fix(android): Erkunden (Apple) overlay back no longer jumps to Home

The iTunes artist / album detail in the Erkunden tab are in-screen overlays on the
NavHost "erkunden" destination, not real routes. Their BackHandler lost back priority
to the NavHost, so one back press popped Erkunden → Home instead of dismissing the
overlay (artist→search, album→artist).

Handle the overlay's back at the app root via OnBackPressedCallback (the same proven
mechanism as the NowPlaying overlay): ExploreScreen reports its current overlay close
action up through onOverlayBackChange, and AppNavigation registers a root-level back
callback for it. Also covers an album opened straight from search (no artist), which
had no back handling at all. Removes the temporary nav-debug strip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -96,6 +96,10 @@ fun AppNavigation() {
     var showNotifications  by remember { mutableStateOf(false) }
     var selectedAbsItem by remember { mutableStateOf<AbsLibraryItem?>(null) }
     var showAbsPlayer   by remember { mutableStateOf(false) }
+    // Close action for the Erkunden (Apple) in-screen overlay (iTunes artist / album). Handled at the
+    // app root because a BackHandler inside the NavHost "erkunden" destination loses back priority to
+    // the NavHost, which pops Erkunden → Home instead of dismissing the overlay.
+    var exploreOverlayBack by remember { mutableStateOf<(() -> Unit)?>(null) }
 
     val backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher
     DisposableEffect(showNowPlaying, backDispatcher) {
@@ -122,6 +126,15 @@ fun AppNavigation() {
         backDispatcher.addCallback(cb)
         onDispose { cb.remove() }
     }
+    DisposableEffect(exploreOverlayBack, backDispatcher) {
+        val action = exploreOverlayBack
+        if (action == null || backDispatcher == null) return@DisposableEffect onDispose {}
+        val cb = object : OnBackPressedCallback(true) {
+            override fun handleOnBackPressed() { action() }
+        }
+        backDispatcher.addCallback(cb)
+        onDispose { cb.remove() }
+    }
 
     fun coverUrl(coverId: String?) = repo.coverArtUrl(coverId, 600)
     fun heroCoverUrl(coverId: String?) = repo.coverArtUrl(coverId, 1000)
@@ -275,6 +288,7 @@ fun AppNavigation() {
                         onAlbumClick  = { album -> navController.navigate("album/${album.id}") },
                         onArtistClick = { artist -> navController.navigate("artist/${artist.id}") },
                         onShowContextMenu = ::showContextMenu,
+                        onOverlayBackChange = { exploreOverlayBack = it },
                         onOpenImporter = { navController.navigate("importer") },
                     )
                 }
@@ -395,21 +409,6 @@ fun AppNavigation() {
             }
         }
 
-        // ── TEMP NAV DEBUG — zeigt den Back-Stack; wird nach der Diagnose entfernt ──
-        val dbgBackStack by navController.currentBackStack.collectAsStateWithLifecycle()
-        Box(Modifier.fillMaxWidth().statusBarsPadding(), contentAlignment = androidx.compose.ui.Alignment.TopCenter) {
-            Text(
-                "🧭 " + dbgBackStack.mapNotNull { it.destination.route?.substringBefore("/") }
-                    .filter { it != "null" }.joinToString(" › "),
-                color = Color(0xFFFFEB3B),
-                fontSize = 9.sp,
-                fontWeight = FontWeight.Bold,
-                modifier = Modifier
-                    .background(Color(0xCC000000))
-                    .padding(horizontal = 6.dp, vertical = 2.dp),
-            )
-        }
-
         // NowPlaying overlay
         AnimatedVisibility(
             visible = showNowPlaying,

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

@@ -1,6 +1,5 @@
 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.*
@@ -50,6 +49,7 @@ fun ExploreScreen(
     onAlbumClick: (com.novasonic.app.api.model.Album) -> Unit,
     onArtistClick: (com.novasonic.app.api.model.Artist) -> Unit,
     onShowContextMenu: ((com.novasonic.app.api.model.Song) -> Unit)? = null,
+    onOverlayBackChange: ((() -> Unit)?) -> Unit = {},
     onOpenImporter: () -> Unit = {},
 ) {
     val vm: ExploreViewModel = viewModel()
@@ -57,10 +57,29 @@ 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() }
+    // Overlay back is handled at the app root (see AppNavigation.exploreOverlayBack): a BackHandler
+    // inside this NavHost destination loses back priority to the NavHost, which pops Erkunden → Home.
+    // Report the current overlay's close action up; clear it when this screen leaves composition
+    // (e.g. drilling into a real album route) so that screen's own back works normally.
+    val overlayKind = when {
+        state.artistDetail != null && state.partialAlbum != null                             -> 4  // artist → partial album
+        state.artistDetail != null && state.albumTracks != null                              -> 3  // artist → iTunes album
+        state.artistDetail != null                                                           -> 2  // iTunes artist
+        state.albumTracks != null                                                            -> 1  // album straight from search
+        else                                                                                 -> 0
+    }
+    DisposableEffect(overlayKind) {
+        onOverlayBackChange(
+            when (overlayKind) {
+                4 -> vm::dismissPartialAlbum
+                3 -> vm::dismissAlbum
+                2 -> vm::closeArtistDetail
+                1 -> vm::dismissAlbum
+                else -> null
+            }
+        )
+        onDispose { onOverlayBackChange(null) }
+    }
 
     // Download sheet
     state.downloadSheetItem?.let { item ->