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/lib/services/api_service.dart

@@ -42,4 +42,12 @@ class ApiService {
       headers: _headers,
     );
   }
+
+  Future<http.Response> changePassword(String oldPassword, String newPassword) async {
+    return await http.post(
+      Uri.parse("${settings.baseUrl}/api/user/change-password"),
+      headers: _headers,
+      body: jsonEncode({"old_password": oldPassword, "new_password": newPassword}),
+    );
+  }
 }