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/chat_service.dart

@@ -7,8 +7,15 @@ class ChatMessage {
   final String username;
   final String content;
   final String roomId;
+  final bool isHistory;
 
-  ChatMessage({required this.type, required this.username, required this.content, required this.roomId});
+  ChatMessage({
+    required this.type,
+    required this.username,
+    required this.content,
+    required this.roomId,
+    this.isHistory = false,
+  });
 
   factory ChatMessage.fromJson(Map<String, dynamic> json) {
     return ChatMessage(
@@ -16,6 +23,7 @@ class ChatMessage {
       username: json['username'] ?? 'Anonymous',
       content: json['content'] ?? '',
       roomId: json['room_id'] ?? '',
+      isHistory: json['is_history'] ?? false,
     );
   }
 
@@ -24,6 +32,7 @@ class ChatMessage {
     'username': username,
     'content': content,
     'room_id': roomId,
+    'is_history': isHistory,
   };
 }