feat: back navigation, NowPlaying fix, home refresh, genre top-6

- ExploreScreen: add BackHandlers for state-based overlays (partialAlbum →
  albumTracks → artistDetail hierarchy) so system back navigates correctly
- AppNavigation: disable NavController's OnBackPressedCallback while NowPlaying
  is open so our BackHandler always intercepts first (fixes tab-switch-then-back bug)
- Home tab: tapping Home while already on Home triggers vm.load() refresh via refreshKey
- HomeViewModel: load all genres sorted by songCount desc (removed take(12) cap)
- HomeScreen: show only top-6 genres + 'Alle anzeigen' button with total count
- New GenreListScreen + GenreViewModel: full scrollable genre list at 'genres' route

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

android/app/src/main/kotlin/com/novasonic/app/ui/AppNavigation.kt

@@ -68,6 +68,13 @@ fun AppNavigation() {
     val playerState by player.state.collectAsStateWithLifecycle()
     val navController = rememberNavController()
     var showNowPlaying by remember { mutableStateOf(false) }
+
+    // Disable NavController's own back-stack handler while NowPlaying is open so
+    // our BackHandler below takes priority instead of it popping tabs first.
+    LaunchedEffect(showNowPlaying) {
+        navController.enableOnBackPressed(!showNowPlaying)
+    }
+
     var contextMenuSong     by remember { mutableStateOf<Song?>(null) }
     var playlistPickerSong  by remember { mutableStateOf<Song?>(null) }
     var infoSheetSong       by remember { mutableStateOf<Song?>(null) }
@@ -81,6 +88,8 @@ fun AppNavigation() {
 
     fun showContextMenu(song: Song) { contextMenuSong = song }
 
+    var homeRefreshKey by remember { mutableIntStateOf(0) }
+
     val currentBackEntry by navController.currentBackStackEntryAsState()
     val currentRoute = currentBackEntry?.destination?.route
     val isTopLevel = TopLevel.entries.any { it.route == currentRoute }
@@ -116,10 +125,14 @@ fun AppNavigation() {
                                 NavigationBarItem(
                                     selected = currentRoute == dest.route,
                                     onClick  = {
+                                        if (dest == TopLevel.HOME && currentRoute == TopLevel.HOME.route) {
+                                            homeRefreshKey++
+                                        } else {
                                             navController.navigate(dest.route) {
                                                 popUpTo(navController.graph.startDestinationId) { saveState = true }
                                                 launchSingleTop = true; restoreState = true
                                             }
+                                        }
                                     },
                                     icon  = { Icon(dest.icon, contentDescription = dest.label) },
                                     label = {
@@ -178,6 +191,8 @@ fun AppNavigation() {
                                 if (songs.isNotEmpty()) player.playSongs(songs.shuffled(), 0)
                             }}
                         },
+                        onShowAllGenres  = { navController.navigate("genres") },
+                        refreshKey       = homeRefreshKey,
                     )
                 }
                 composable(TopLevel.LIBRARY.route) {
@@ -268,6 +283,18 @@ fun AppNavigation() {
                         onBack          = { navController.popBackStack() },
                     )
                 }
+                composable("genres") {
+                    GenreListScreen(
+                        onGenreClick = { genre -> navController.navigate("genre/${encodeRoute(genre.value)}") },
+                        onGenrePlay  = { genre ->
+                            scope.launch { runCatching {
+                                val songs = repo.getSongsByGenre(genre.value)
+                                if (songs.isNotEmpty()) player.playSongs(songs.shuffled(), 0)
+                            }}
+                        },
+                        onBack = { navController.popBackStack() },
+                    )
+                }
             }
         }
 

android/app/src/main/kotlin/com/novasonic/app/ui/screen/ExploreScreen.kt

@@ -1,5 +1,6 @@
 package com.novasonic.app.ui.screen
 
+import androidx.activity.compose.BackHandler
 import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.*
@@ -55,6 +56,11 @@ fun ExploreScreen(
     val focusManager = LocalFocusManager.current
     val context = LocalContext.current
 
+    // BackHandlers for state-based navigation (must be unconditional)
+    BackHandler(enabled = state.artistDetail != null && state.partialAlbum != null) { vm.dismissPartialAlbum() }
+    BackHandler(enabled = state.artistDetail != null && state.albumTracks != null && state.partialAlbum == null) { vm.dismissAlbum() }
+    BackHandler(enabled = state.artistDetail != null && state.albumTracks == null && state.partialAlbum == null) { vm.closeArtistDetail() }
+
     // Download sheet
     state.downloadSheetItem?.let { item ->
         ItunesDownloadSheet(

android/app/src/main/kotlin/com/novasonic/app/ui/screen/GenreListScreen.kt

@@ -0,0 +1,96 @@
+package com.novasonic.app.ui.screen
+
+import androidx.compose.foundation.layout.*
+import androidx.compose.foundation.lazy.LazyColumn
+import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
+import androidx.compose.material3.*
+import androidx.compose.runtime.*
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import androidx.lifecycle.viewmodel.compose.viewModel
+import com.novasonic.app.api.model.Genre
+import com.novasonic.app.ui.theme.*
+import com.novasonic.app.viewmodel.GenreViewModel
+
+@Composable
+fun GenreListScreen(
+    onGenreClick: (Genre) -> Unit,
+    onGenrePlay: (Genre) -> Unit,
+    onBack: () -> Unit,
+) {
+    val vm: GenreViewModel = viewModel()
+    val state by vm.state.collectAsStateWithLifecycle()
+
+    Column(Modifier.fillMaxSize()) {
+        Row(
+            verticalAlignment = Alignment.CenterVertically,
+            modifier = Modifier.padding(start = 4.dp, end = 16.dp, top = 8.dp, bottom = 4.dp),
+        ) {
+            IconButton(onClick = onBack) {
+                Icon(Icons.AutoMirrored.Filled.ArrowBack, "Zurück", tint = NsOnSurface2)
+            }
+            Text(
+                "Alle Genres",
+                fontFamily = Outfit,
+                fontWeight = FontWeight.Bold,
+                fontSize = 18.sp,
+                color = NsOnSurface,
+            )
+        }
+
+        when {
+            state.isLoading -> {
+                Box(Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
+                    CircularProgressIndicator(color = NsAccent)
+                }
+            }
+            state.error != null -> {
+                Column(
+                    horizontalAlignment = Alignment.CenterHorizontally,
+                    modifier = Modifier.fillMaxSize().padding(32.dp),
+                ) {
+                    Text(state.error!!, color = MaterialTheme.colorScheme.error,
+                        style = MaterialTheme.typography.bodyMedium)
+                    Spacer(Modifier.height(12.dp))
+                    Button(onClick = { vm.load() },
+                        colors = ButtonDefaults.buttonColors(containerColor = NsAccent)) {
+                        Text("Retry")
+                    }
+                }
+            }
+            else -> {
+                LazyColumn(
+                    contentPadding = PaddingValues(bottom = 24.dp),
+                    modifier = Modifier.fillMaxSize(),
+                ) {
+                    val chunked = state.genres.chunked(2)
+                    chunked.forEachIndexed { rowIdx, rowGenres ->
+                        item(key = "row_$rowIdx") {
+                            Row(
+                                horizontalArrangement = Arrangement.spacedBy(8.dp),
+                                modifier = Modifier.padding(horizontal = 16.dp, vertical = 4.dp),
+                            ) {
+                                rowGenres.forEachIndexed { colIdx, genre ->
+                                    val colorIdx = (rowIdx * 2 + colIdx) % genreColors.size
+                                    GenreChip(
+                                        genre    = genre,
+                                        bgColor  = genreColors[colorIdx],
+                                        onClick  = { onGenreClick(genre) },
+                                        onPlay   = { onGenrePlay(genre) },
+                                        modifier = Modifier.weight(1f),
+                                    )
+                                }
+                                if (rowGenres.size == 1) Spacer(Modifier.weight(1f))
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}

android/app/src/main/kotlin/com/novasonic/app/ui/screen/HomeScreen.kt

@@ -32,7 +32,7 @@ import com.novasonic.app.ui.theme.*
 import com.novasonic.app.viewmodel.HomeViewModel
 import java.util.Calendar
 
-private val genreColors = listOf(
+internal val genreColors = listOf(
     Color(0xFF1a1a3e), Color(0xFF1e2a1a), Color(0xFF2a1a1a),
     Color(0xFF1a2a2a), Color(0xFF2a1e1a), Color(0xFF1e1a2a),
     Color(0xFF2a2a1a), Color(0xFF1a2a1e), Color(0xFF201520),
@@ -46,10 +46,14 @@ fun HomeScreen(
     onAlbumPlay: (Album) -> Unit,
     onGenreClick: (Genre) -> Unit,
     onGenrePlay: ((Genre) -> Unit)? = null,
+    onShowAllGenres: (() -> Unit)? = null,
+    refreshKey: Int = 0,
 ) {
     val vm: HomeViewModel = viewModel()
     val state by vm.state.collectAsStateWithLifecycle()
 
+    LaunchedEffect(refreshKey) { if (refreshKey > 0) vm.load() }
+
     val greeting = remember {
         val hour = Calendar.getInstance().get(Calendar.HOUR_OF_DAY)
         when {
@@ -118,8 +122,28 @@ fun HomeScreen(
         if (state.genres.isNotEmpty()) {
             item { SectionHeader("Genres") }
             item {
-                GenreGrid(genres = state.genres, onGenreClick = onGenreClick, onGenrePlay = onGenrePlay)
+                GenreGrid(genres = state.genres.take(6), onGenreClick = onGenreClick, onGenrePlay = onGenrePlay)
-                Spacer(Modifier.height(28.dp))
+                if (onShowAllGenres != null && state.genres.size > 6) {
+                    Box(
+                        contentAlignment = Alignment.Center,
+                        modifier = Modifier
+                            .fillMaxWidth()
+                            .padding(horizontal = 16.dp, vertical = 10.dp)
+                            .clip(RoundedCornerShape(10.dp))
+                            .background(NsSurface)
+                            .clickable(onClick = onShowAllGenres)
+                            .padding(vertical = 12.dp),
+                    ) {
+                        Text(
+                            "Alle anzeigen  ·  ${state.genres.size}",
+                            fontFamily = Outfit,
+                            fontWeight = FontWeight.SemiBold,
+                            fontSize = 13.sp,
+                            color = NsAccent,
+                        )
+                    }
+                }
+                Spacer(Modifier.height(20.dp))
             }
         }
 
@@ -238,7 +262,7 @@ private fun HeroCard(album: Album, coverUrl: String?, onPlay: () -> Unit, onClic
 }
 
 @Composable
-private fun GenreGrid(
+internal fun GenreGrid(
     genres: List<Genre>,
     onGenreClick: (Genre) -> Unit,
     onGenrePlay: ((Genre) -> Unit)?,
@@ -267,7 +291,7 @@ private fun GenreGrid(
 }
 
 @Composable
-private fun GenreChip(
+internal fun GenreChip(
     genre: Genre,
     bgColor: Color,
     onClick: () -> Unit,

android/app/src/main/kotlin/com/novasonic/app/viewmodel/GenreViewModel.kt

@@ -0,0 +1,33 @@
+package com.novasonic.app.viewmodel
+
+import android.app.Application
+import androidx.lifecycle.AndroidViewModel
+import androidx.lifecycle.viewModelScope
+import com.novasonic.app.api.model.Genre
+import com.novasonic.app.app
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.launch
+
+data class GenreUiState(
+    val genres: List<Genre> = emptyList(),
+    val isLoading: Boolean = true,
+    val error: String? = null,
+)
+
+class GenreViewModel(app: Application) : AndroidViewModel(app) {
+    private val repo = app.app.repository
+    private val _state = MutableStateFlow(GenreUiState())
+    val state: StateFlow<GenreUiState> = _state
+
+    init { load() }
+
+    fun load() {
+        viewModelScope.launch {
+            _state.value = GenreUiState(isLoading = true)
+            runCatching { repo.getGenres().filter { it.songCount > 0 }.sortedByDescending { it.songCount } }
+                .onSuccess { _state.value = GenreUiState(genres = it) }
+                .onFailure { _state.value = GenreUiState(error = it.message) }
+        }
+    }
+}

android/app/src/main/kotlin/com/novasonic/app/viewmodel/HomeViewModel.kt

@@ -61,7 +61,7 @@ class HomeViewModel(app: Application) : AndroidViewModel(app) {
                     recentAlbums   = recent,
                     frequentAlbums = frequent,
                     randomAlbums   = random,
-                    genres         = genres.filter { it.songCount > 0 }.take(12),
+                    genres         = genres.filter { it.songCount > 0 }.sortedByDescending { it.songCount },
                     isLoading      = false,
                     error          = null,
                 )