perf: lazy extension VM init, incremental startup maintenance, and UI optimizations

- Defer extension VM initialization until first use with lockReadyVM() pattern to eliminate TOCTOU races and reduce startup overhead
- Add validateExtensionLoad() to catch JS errors at install time without keeping VM alive
- Teardown VM on extension disable to free resources; re-init lazily on re-enable
- Replace full orphan cleanup with incremental cursor-based pagination across launches
- Batch DB writes (upsertBatch, replaceAll) with transactions for atomicity
- Parse JSON natively on Kotlin side to avoid double-serialization over MethodChannel
- Add identity-based memoization caches for unified items and path match keys in queue tab
- Use ValueListenableBuilder for targeted embedded cover refreshes instead of full setState
- Extract shared widgets (_buildAlbumGridItemCore, _buildFilterButton, _navigateWithUnfocus)
- Use libraryCollectionsProvider selector and MediaQuery.paddingOf for fewer rebuilds
- Simplify supporter chip tiers and localize remaining hardcoded strings

lib/services/library_database.dart

@@ -255,20 +255,41 @@ class LibraryDatabase {
   Future<void> upsertBatch(List<Map<String, dynamic>> items) async {
     if (items.isEmpty) return;
     final db = await database;
-    final batch = db.batch();
-
-    for (final json in items) {
-      batch.insert(
-        'library',
-        _jsonToDbRow(json),
-        conflictAlgorithm: ConflictAlgorithm.replace,
-      );
-    }
-
-    await batch.commit(noResult: true);
+    await db.transaction((txn) async {
+      final batch = txn.batch();
+      for (final json in items) {
+        batch.insert(
+          'library',
+          _jsonToDbRow(json),
+          conflictAlgorithm: ConflictAlgorithm.replace,
+        );
+      }
+      await batch.commit(noResult: true);
+    });
     _log.i('Batch inserted ${items.length} items');
   }
 
+  Future<void> replaceAll(List<Map<String, dynamic>> items) async {
+    final db = await database;
+    await db.transaction((txn) async {
+      await txn.delete('library');
+      if (items.isEmpty) {
+        return;
+      }
+
+      final batch = txn.batch();
+      for (final json in items) {
+        batch.insert(
+          'library',
+          _jsonToDbRow(json),
+          conflictAlgorithm: ConflictAlgorithm.replace,
+        );
+      }
+      await batch.commit(noResult: true);
+    });
+    _log.i('Replaced library with ${items.length} items');
+  }
+
   Future<List<Map<String, dynamic>>> getAll({int? limit, int? offset}) async {
     final db = await database;
     final rows = await db.query(