feat: implement mobile AI chat feature and fix duplicate response issue (#610)

Backend fixes:
- Fix duplicate AssistantResponseJob triggering causing duplicate AI responses
- UserMessage model already handles job triggering via after_create_commit callback
- Remove redundant job enqueue in chats_controller and messages_controller

Mobile app features:
- Implement complete AI chat interface and conversation management
- Add Chat, Message, and ToolCall data models
- Add ChatProvider for state management with polling mechanism
- Add ChatService to handle all chat-related API requests
- Add chat list screen (ChatListScreen)
- Add conversation detail screen (ChatConversationScreen)
- Refactor navigation structure with bottom navigation bar (MainNavigationScreen)
- Add settings screen (SettingsScreen)
- Optimize TransactionsProvider to support account filtering

Technical details:
- Implement message polling mechanism for real-time AI responses
- Support chat creation, deletion, retry and other operations
- Integrate Material Design 3 design language
- Improve user experience and error handling

Co-authored-by: dwvwdv <dwvwdv@protonmail.com>

mobile/lib/models/chat.dart

@@ -0,0 +1,77 @@
+import 'message.dart';
+
+class Chat {
+  final String id;
+  final String title;
+  final String? error;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+  final List<Message> messages;
+  final int? messageCount;
+  final DateTime? lastMessageAt;
+
+  Chat({
+    required this.id,
+    required this.title,
+    this.error,
+    required this.createdAt,
+    required this.updatedAt,
+    this.messages = const [],
+    this.messageCount,
+    this.lastMessageAt,
+  });
+
+  factory Chat.fromJson(Map<String, dynamic> json) {
+    return Chat(
+      id: json['id'].toString(),
+      title: json['title'] as String,
+      error: json['error'] as String?,
+      createdAt: DateTime.parse(json['created_at'] as String),
+      updatedAt: DateTime.parse(json['updated_at'] as String),
+      messages: json['messages'] != null
+          ? (json['messages'] as List)
+              .map((m) => Message.fromJson(m as Map<String, dynamic>))
+              .toList()
+          : [],
+      messageCount: json['message_count'] as int?,
+      lastMessageAt: json['last_message_at'] != null
+          ? DateTime.parse(json['last_message_at'] as String)
+          : null,
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'title': title,
+      'error': error,
+      'created_at': createdAt.toIso8601String(),
+      'updated_at': updatedAt.toIso8601String(),
+      'messages': messages.map((m) => m.toJson()).toList(),
+      'message_count': messageCount,
+      'last_message_at': lastMessageAt?.toIso8601String(),
+    };
+  }
+
+  Chat copyWith({
+    String? id,
+    String? title,
+    String? error,
+    DateTime? createdAt,
+    DateTime? updatedAt,
+    List<Message>? messages,
+    int? messageCount,
+    DateTime? lastMessageAt,
+  }) {
+    return Chat(
+      id: id ?? this.id,
+      title: title ?? this.title,
+      error: error ?? this.error,
+      createdAt: createdAt ?? this.createdAt,
+      updatedAt: updatedAt ?? this.updatedAt,
+      messages: messages ?? this.messages,
+      messageCount: messageCount ?? this.messageCount,
+      lastMessageAt: lastMessageAt ?? this.lastMessageAt,
+    );
+  }
+}

mobile/lib/models/message.dart

@@ -0,0 +1,56 @@
+import 'tool_call.dart';
+
+class Message {
+  final String id;
+  final String type;
+  final String role;
+  final String content;
+  final String? model;
+  final DateTime createdAt;
+  final DateTime updatedAt;
+  final List<ToolCall>? toolCalls;
+
+  Message({
+    required this.id,
+    required this.type,
+    required this.role,
+    required this.content,
+    this.model,
+    required this.createdAt,
+    required this.updatedAt,
+    this.toolCalls,
+  });
+
+  factory Message.fromJson(Map<String, dynamic> json) {
+    return Message(
+      id: json['id'].toString(),
+      type: json['type'] as String,
+      role: json['role'] as String,
+      content: json['content'] as String,
+      model: json['model'] as String?,
+      createdAt: DateTime.parse(json['created_at'] as String),
+      updatedAt: DateTime.parse(json['updated_at'] as String),
+      toolCalls: json['tool_calls'] != null
+          ? (json['tool_calls'] as List)
+              .map((tc) => ToolCall.fromJson(tc as Map<String, dynamic>))
+              .toList()
+          : null,
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'type': type,
+      'role': role,
+      'content': content,
+      'model': model,
+      'created_at': createdAt.toIso8601String(),
+      'updated_at': updatedAt.toIso8601String(),
+      'tool_calls': toolCalls?.map((tc) => tc.toJson()).toList(),
+    };
+  }
+
+  bool get isUser => role == 'user';
+  bool get isAssistant => role == 'assistant';
+}

mobile/lib/models/tool_call.dart

@@ -0,0 +1,53 @@
+import 'dart:convert';
+
+class ToolCall {
+  final String id;
+  final String functionName;
+  final Map<String, dynamic> functionArguments;
+  final Map<String, dynamic>? functionResult;
+  final DateTime createdAt;
+
+  ToolCall({
+    required this.id,
+    required this.functionName,
+    required this.functionArguments,
+    this.functionResult,
+    required this.createdAt,
+  });
+
+  factory ToolCall.fromJson(Map<String, dynamic> json) {
+    return ToolCall(
+      id: json['id'].toString(),
+      functionName: json['function_name'] as String,
+      functionArguments: _parseJsonField(json['function_arguments']),
+      functionResult: json['function_result'] != null
+          ? _parseJsonField(json['function_result'])
+          : null,
+      createdAt: DateTime.parse(json['created_at'] as String),
+    );
+  }
+
+  static Map<String, dynamic> _parseJsonField(dynamic field) {
+    if (field == null) return {};
+    if (field is Map<String, dynamic>) return field;
+    if (field is String) {
+      try {
+        final parsed = jsonDecode(field);
+        return parsed is Map<String, dynamic> ? parsed : {};
+      } catch (e) {
+        return {};
+      }
+    }
+    return {};
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'function_name': functionName,
+      'function_arguments': functionArguments,
+      'function_result': functionResult,
+      'created_at': createdAt.toIso8601String(),
+    };
+  }
+}