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/pubspec.lock

@@ -197,6 +197,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.1.3"
+  google_fonts:
+    dependency: "direct main"
+    description:
+      name: google_fonts
+      sha256: ba03d03bcaa2f6cb7bd920e3b5027181db75ab524f8891c8bc3aa603885b8055
+      url: "https://pub.dev"
+    source: hosted
+    version: "6.3.3"
   hooks:
     dependency: transitive
     description: