feat(ui): 添加明暗主题切换支持

- 创建 ThemeProvider 管理主题状态
- 配置浅色和深色主题（Vercel/Linear 风格）
- 集成 Google Fonts（Inter + JetBrains Mono）
- 在我的页面添加主题切换开关
- 更新颜色系统符合 modernization-v2.md 规范
- 优化间距和圆角系统

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flutter_monisuo/lib/ui/pages/mine/mine_page.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:shadcn_ui/shadcn_ui.dart';
 import 'package:provider/provider.dart';
 import '../../../providers/auth_provider.dart';
+import '../../../providers/theme_provider.dart';
 import '../auth/login_page.dart';
 
 /// 菜单项数据模型
@@ -241,15 +242,19 @@ class _MenuList extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final theme = ShadTheme.of(context);
-    final items = _buildMenuItems();
+    final themeProvider = context.watch<ThemeProvider>();
 
     return ShadCard(
       padding: EdgeInsets.zero,
       child: Column(
         children: [
-          for (var i = 0; i < items.length; i++) ...[
-            _MenuItemTile(item: items[i]),
-            if (i < items.length - 1)
+          // 主题切换开关（特殊处理）
+          _ThemeToggleTile(isDarkMode: themeProvider.isDarkMode),
+          Divider(color: theme.colorScheme.border, height: 1, indent: 56),
+          // 普通菜单项
+          for (var i = 0; i < _buildMenuItems().length; i++) ...[
+            _MenuItemTile(item: _buildMenuItems()[i]),
+            if (i < _buildMenuItems().length - 1)
               Divider(color: theme.colorScheme.border, height: 1, indent: 56),
           ],
         ],
@@ -268,6 +273,51 @@ class _MenuList extends StatelessWidget {
   }
 }
 
+/// 主题切换组件
+class _ThemeToggleTile extends StatelessWidget {
+  final bool isDarkMode;
+
+  const _ThemeToggleTile({required this.isDarkMode});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = ShadTheme.of(context);
+    final themeProvider = context.read<ThemeProvider>();
+
+    return InkWell(
+      onTap: () => themeProvider.toggleTheme(),
+      child: Padding(
+        padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 12),
+        child: Row(
+          children: [
+            _MenuIcon(icon: isDarkMode ? LucideIcons.moon : LucideIcons.sun),
+            const SizedBox(width: 12),
+            Expanded(
+              child: Column(
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text('深色模式', style: theme.textTheme.small.copyWith(fontWeight: FontWeight.w500)),
+                  const SizedBox(height: 2),
+                  Text(
+                    isDarkMode ? '当前：深色主题' : '当前：浅色主题',
+                    style: theme.textTheme.muted.copyWith(fontSize: 11),
+                  ),
+                ],
+              ),
+            ),
+            Switch(
+              value: isDarkMode,
+              onChanged: (_) => themeProvider.toggleTheme(),
+              activeTrackColor: theme.colorScheme.primary.withValues(alpha: 0.5),
+              activeColor: theme.colorScheme.primary,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
 /// 菜单项组件
 class _MenuItemTile extends StatelessWidget {
   final _MenuItem item;