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）

prisma/migrations/20260308142357_add_folder_visibility_and_favorites/migration.sql

@@ -0,0 +1,33 @@
+-- CreateEnum
+CREATE TYPE "Visibility" AS ENUM ('PRIVATE', 'PUBLIC');
+
+-- AlterTable
+ALTER TABLE "folders" ADD COLUMN "visibility" "Visibility" NOT NULL DEFAULT 'PRIVATE';
+
+-- CreateTable
+CREATE TABLE "folder_favorites" (
+    "id" SERIAL NOT NULL,
+    "user_id" TEXT NOT NULL,
+    "folder_id" INTEGER NOT NULL,
+    "created_at" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "folder_favorites_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "folder_favorites_folder_id_idx" ON "folder_favorites"("folder_id");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "folder_favorites_user_id_folder_id_key" ON "folder_favorites"("user_id", "folder_id");
+
+-- CreateIndex
+CREATE INDEX "folder_favorites_user_id_idx" ON "folder_favorites"("user_id");
+
+-- CreateIndex
+CREATE INDEX "folders_visibility_idx" ON "folders"("visibility");
+
+-- AddForeignKey
+ALTER TABLE "folder_favorites" ADD CONSTRAINT "folder_favorites_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "user"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "folder_favorites" ADD CONSTRAINT "folder_favorites_folder_id_fkey" FOREIGN KEY ("folder_id") REFERENCES "folders"("id") ON DELETE CASCADE ON UPDATE CASCADE;