refactor(apps): 主题系统迁移至 ColorScheme + 扩展架构并支持 Dark Mode

apps/lib/shared/widgets/chat_bubble.dart

@@ -1,6 +1,5 @@
 import 'package:flutter/material.dart';
 import '../../../core/l10n/l10n.dart';
-import '../../../core/theme/design_tokens.dart';
 
 enum MessageSender { user, ai }
 
@@ -23,6 +22,7 @@ class ChatBubble extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final isUser = sender == MessageSender.user;
+    final colorScheme = Theme.of(context).colorScheme;
 
     return Padding(
       padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 6),
@@ -36,7 +36,10 @@ class ChatBubble extends StatelessWidget {
               padding: const EdgeInsets.only(bottom: 6),
               child: Text(
                 _formatTimestamp(timestamp),
-                style: const TextStyle(fontSize: 11, color: AppColors.slate400),
+                style: TextStyle(
+                  fontSize: 11,
+                  color: colorScheme.onSurfaceVariant.withValues(alpha: 0.7),
+                ),
               ),
             ),
           Container(
@@ -45,11 +48,11 @@ class ChatBubble extends StatelessWidget {
             ),
             padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 12),
             decoration: BoxDecoration(
-              color: isUser ? AppColors.blue500 : AppColors.white,
+              color: isUser ? colorScheme.primary : colorScheme.surface,
               borderRadius: BorderRadius.circular(20),
               boxShadow: [
                 BoxShadow(
-                  color: Colors.black.withValues(alpha: 0.06),
+                  color: colorScheme.shadow.withValues(alpha: 0.08),
                   blurRadius: 8,
                   offset: const Offset(0, 2),
                 ),
@@ -60,7 +63,9 @@ class ChatBubble extends StatelessWidget {
                     content,
                     style: TextStyle(
                       fontSize: 15,
-                      color: isUser ? AppColors.white : AppColors.slate700,
+                      color: isUser
+                          ? colorScheme.onPrimary
+                          : colorScheme.onSurface,
                       height: 1.45,
                     ),
                   )