fix: persist download queue to survive app restart (v1.6.1)

- Download queue now persisted to SharedPreferences
- Auto-restore pending downloads on app restart
- Interrupted downloads reset to queued and auto-resumed
- singleTask launch mode to prevent app restart on share intent
- onNewIntent handler for proper intent handling
- Reverted share_plus to 10.1.4 (12.0.1 has Kotlin build issues)

lib/screens/track_metadata_screen.dart

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