feat: implement chat history, theme customization, and password management

- Added chat history persistence for active rooms with auto-cleanup on stream end.
- Overhauled Settings page with user profile, theme color picker, and password change.
- Added backend API for user password updates.
- Integrated flutter_launcher_icons and updated app icon to 'H' logo.
- Fixed 'Duplicate keys' bug in danmaku by using UniqueKey and filtering historical messages.
- Updated version to 1.0.0-beta3.5 and author info.

frontend/test/widget_test.dart

@@ -13,7 +13,7 @@ import 'package:hightube/main.dart';
 void main() {
   testWidgets('Counter increments smoke test', (WidgetTester tester) async {
     // Build our app and trigger a frame.
-    await tester.pumpWidget(const MyApp());
+    await tester.pumpWidget(HightubeApp());
 
     // Verify that our counter starts at 0.
     expect(find.text('0'), findsOneWidget);