feat: background download service + queue persistence (v1.6.1)

- Add foreground service for background downloads with wake lock
- Persist download queue to SharedPreferences for app restart recovery
- Fix share intent causing app restart (singleTask + onNewIntent)
- Fix back button clearing state during loading
- Upgrade Kotlin to 2.3.0 for share_plus 12.0.1 compatibility
- Add WAKE_LOCK permission for foreground service

lib/screens/main_shell.dart

@@ -150,8 +150,9 @@ class _MainShellState extends ConsumerState<MainShell> {
           return;
         }
         
-        // If on Search tab and has text in search bar or has content, clear it
-        if (_currentIndex == 0 && (trackState.hasSearchText || trackState.hasContent || trackState.isLoading)) {
+        // If on Search tab and has text in search bar or has content (but not loading), clear it
+        // Don't clear while loading - this prevents clearing during share intent processing
+        if (_currentIndex == 0 && !trackState.isLoading && (trackState.hasSearchText || trackState.hasContent)) {
           ref.read(trackProvider.notifier).clear();
           return;
         }
@@ -162,6 +163,11 @@ class _MainShellState extends ConsumerState<MainShell> {
           return;
         }
         
+        // If loading, ignore back press
+        if (trackState.isLoading) {
+          return;
+        }
+        
         // Already at root, show exit dialog
         final shouldPop = await _showExitDialog();
         if (shouldPop && context.mounted) {

lib/screens/track_metadata_screen.dart

@@ -938,9 +938,11 @@ class _TrackMetadataScreenState extends ConsumerState<TrackMetadataScreen> {
       return;
     }
     
-    await Share.shareXFiles(
-      [XFile(item.filePath)],
-      text: '${item.trackName} - ${item.artistName}',
+    await SharePlus.instance.share(
+      ShareParams(
+        files: [XFile(item.filePath)],
+        text: '${item.trackName} - ${item.artistName}',
+      ),
     );
   }