perf: optimize polling, progress caching, staggered warmup, and snapshot-based library scan

- Reduce polling interval from 800ms to 1200ms across download progress, library scan, and Android native stream
- Add dirty-flag caching to Go GetMultiProgress() to skip redundant JSON marshaling
- Replace eager provider initialization with staggered Timer-based warmup (400/900/1600ms)
- Add snapshot-based incremental library scan to avoid large MethodChannel payloads
- Move history stats and grouped album filtering to Riverpod providers for better cache invalidation
- Cap home tab history preview to 48 items with deep equality wrapper to reduce rebuilds
- Throttle foreground service notification updates to 2% progress buckets
- Migrate PageView to PageView.builder with AutomaticKeepAliveClientMixin
- Add comparison table to README

lib/services/library_database.dart

@@ -1,3 +1,5 @@
+import 'dart:io';
+
 import 'package:sqflite/sqflite.dart';
 import 'package:path/path.dart';
 import 'package:path_provider/path_provider.dart';
@@ -470,6 +472,34 @@ class LibraryDatabase {
     return result;
   }
 
+  /// Export file modification times to a compact line-based snapshot that
+  /// native code can read without receiving a large method-channel payload.
+  Future<String> writeFileModTimesSnapshot() async {
+    final db = await database;
+    final rows = await db.rawQuery(
+      'SELECT file_path, COALESCE(file_mod_time, 0) AS file_mod_time FROM library',
+    );
+    final tempDir = await getTemporaryDirectory();
+    final file = File(
+      join(
+        tempDir.path,
+        'library_file_mod_times_${DateTime.now().microsecondsSinceEpoch}.tsv',
+      ),
+    );
+    final buffer = StringBuffer();
+    for (final row in rows) {
+      final path = row['file_path'] as String?;
+      if (path == null || path.isEmpty) continue;
+      final modTime = (row['file_mod_time'] as num?)?.toInt() ?? 0;
+      buffer
+        ..write(modTime)
+        ..write('\t')
+        ..writeln(path);
+    }
+    await file.writeAsString(buffer.toString(), flush: true);
+    return file.path;
+  }
+
   /// Update file_mod_time for existing rows using file_path as key.
   Future<void> updateFileModTimes(Map<String, int> fileModTimes) async {
     if (fileModTimes.isEmpty) return;

lib/services/platform_bridge.dart

@@ -1090,6 +1090,17 @@ class PlatformBridge {
     return jsonDecode(result as String) as Map<String, dynamic>;
   }
 
+  static Future<Map<String, dynamic>> scanLibraryFolderIncrementalFromSnapshot(
+    String folderPath,
+    String snapshotPath,
+  ) async {
+    final result = await _channel.invokeMethod(
+      'scanLibraryFolderIncrementalFromSnapshot',
+      {'folder_path': folderPath, 'snapshot_path': snapshotPath},
+    );
+    return jsonDecode(result as String) as Map<String, dynamic>;
+  }
+
   static Future<List<Map<String, dynamic>>> scanSafTree(String treeUri) async {
     _log.i('scanSafTree: $treeUri');
     final result = await _channel.invokeMethod('scanSafTree', {
@@ -1115,6 +1126,17 @@ class PlatformBridge {
     return jsonDecode(result as String) as Map<String, dynamic>;
   }
 
+  static Future<Map<String, dynamic>> scanSafTreeIncrementalFromSnapshot(
+    String treeUri,
+    String snapshotPath,
+  ) async {
+    final result = await _channel.invokeMethod(
+      'scanSafTreeIncrementalFromSnapshot',
+      {'tree_uri': treeUri, 'snapshot_path': snapshotPath},
+    );
+    return jsonDecode(result as String) as Map<String, dynamic>;
+  }
+
   /// Get last-modified timestamps for a list of SAF file URIs.
   /// Returns map uri -> modTime (unix millis), only for files that still exist.
   static Future<Map<String, int>> getSafFileModTimes(List<String> uris) async {