fix: genre loading spinner + NowPlaying back button priority

GenreViewModel: GenreUiState(genres = it) left isLoading=true (default),
so genres loaded but spinner never cleared. Added isLoading=false explicitly.

AppNavigation: replace BackHandler + navController.enableOnBackPressed with a
DisposableEffect that re-adds an OnBackPressedCallback each time NowPlaying
opens. Since callbacks are LIFO, a freshly-added callback always beats the
BackHandler NavHost registers internally, regardless of composition order.

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

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

@@ -1,6 +1,7 @@
 package com.novasonic.app.ui
 
-import androidx.activity.compose.BackHandler
+import androidx.activity.OnBackPressedCallback
+import androidx.activity.compose.LocalOnBackPressedDispatcherOwner
 import androidx.compose.animation.*
 import androidx.compose.animation.core.FastOutSlowInEasing
 import androidx.compose.animation.core.tween
@@ -69,10 +70,17 @@ fun AppNavigation() {
     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)
+    // Re-register a fresh callback each time NowPlaying opens so it is always
+    // the LAST entry in the dispatcher queue (LIFO = highest priority), beating
+    // the BackHandler that NavHost registers internally for its own back stack.
+    val backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher
+    DisposableEffect(showNowPlaying, backDispatcher) {
+        if (!showNowPlaying || backDispatcher == null) return@DisposableEffect onDispose {}
+        val cb = object : OnBackPressedCallback(true) {
+            override fun handleOnBackPressed() { showNowPlaying = false }
+        }
+        backDispatcher.addCallback(cb)
+        onDispose { cb.remove() }
     }
 
     var contextMenuSong     by remember { mutableStateOf<Song?>(null) }
@@ -298,9 +306,6 @@ fun AppNavigation() {
             }
         }
 
-        // Dismiss NowPlaying with hardware back button
-        BackHandler(enabled = showNowPlaying) { showNowPlaying = false }
-
         // NowPlaying overlay — outside Scaffold so WindowInsets are not double-consumed
         AnimatedVisibility(
             visible = showNowPlaying,

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

@@ -26,8 +26,8 @@ class GenreViewModel(app: Application) : AndroidViewModel(app) {
         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) }
+                .onSuccess { _state.value = GenreUiState(genres = it, isLoading = false) }
+                .onFailure { _state.value = GenreUiState(error = it.message, isLoading = false) }
         }
     }
 }