feat(folders): 完善公开文件夹功能 - 添加 /explore 和 /favorites 页面

- 新增 /explore 页面：浏览和搜索公开文件夹
- 新增 /explore/[id] 页面：以只读模式查看公开文件夹
- 新增 /favorites 页面：管理收藏的文件夹
- 重构 /folders 页面：仅显示当前用户的文件夹
- 更新导航栏：添加 Explore 和 Favorites 链接
- 添加 i18n 翻译：explore 和 favorites 相关文本
- 更新 AGENTS.md：添加数据库迁移规范（必须使用 migrate dev）

messages/en-US.json

@@ -159,7 +159,9 @@
     "sourceCode": "GitHub",
     "sign_in": "Sign In",
     "profile": "Profile",
-    "folders": "Folders"
+    "folders": "Folders",
+    "explore": "Explore",
+    "favorites": "Favorites"
   },
   "profile": {
     "myProfile": "My Profile",
@@ -255,6 +257,26 @@
     "savedToFolder": "Saved to folder: {folderName}",
     "saveFailed": "Save failed, please try again later"
   },
+  "explore": {
+    "title": "Explore",
+    "subtitle": "Discover public folders",
+    "searchPlaceholder": "Search public folders...",
+    "loading": "Loading...",
+    "noFolders": "No public folders found",
+    "folderInfo": "{userName} • {totalPairs} pairs",
+    "unknownUser": "Unknown User",
+    "favorite": "Favorite",
+    "unfavorite": "Unfavorite",
+    "pleaseLogin": "Please login first"
+  },
+  "favorites": {
+    "title": "My Favorites",
+    "subtitle": "Folders you've favorited",
+    "loading": "Loading...",
+    "noFavorites": "No favorites yet",
+    "folderInfo": "{userName} • {totalPairs} pairs",
+    "unknownUser": "Unknown User"
+  },
   "user_profile": {
     "anonymous": "Anonymous",
     "email": "Email",

messages/zh-CN.json

@@ -159,7 +159,9 @@
     "sourceCode": "源码",
     "sign_in": "登录",
     "profile": "个人资料",
-    "folders": "文件夹"
+    "folders": "文件夹",
+    "explore": "探索",
+    "favorites": "收藏"
   },
   "profile": {
     "myProfile": "我的个人资料",
@@ -255,6 +257,34 @@
     "savedToFolder": "已保存到文件夹：{folderName}",
     "saveFailed": "保存失败，请稍后重试"
   },
+  "explore": {
+    "title": "探索",
+    "subtitle": "发现公开文件夹",
+    "searchPlaceholder": "搜索公开文件夹...",
+    "loading": "加载中...",
+    "noFolders": "没有找到公开文件夹",
+    "folderInfo": "{userName} • {totalPairs} 个文本对",
+    "unknownUser": "未知用户",
+    "favorite": "收藏",
+    "unfavorite": "取消收藏",
+    "pleaseLogin": "请先登录"
+  },
+  "favorites": {
+    "title": "收藏",
+    "subtitle": "我收藏的文件夹",
+    "loading": "加载中...",
+    "noFavorites": "还没有收藏",
+    "folderInfo": "{userName} • {totalPairs} 个文本对",
+    "unknownUser": "未知用户"
+  },
+  "favorites": {
+    "title": "我的收藏",
+    "subtitle": "收藏的公开文件夹",
+    "loading": "加载中...",
+    "noFavorites": "还没有收藏任何文件夹",
+    "folderInfo": "{userName} • {totalPairs} 个文本对",
+    "unknownUser": "未知用户"
+  },
   "user_profile": {
     "anonymous": "匿名",
     "email": "邮箱",