refactor: 完全重构为 Anki 兼容数据结构

- 用 Deck 替换 Folder
- 用 Note + Card 替换 Pair (双向复习)
- 添加 NoteType (卡片模板)
- 添加 Revlog (复习历史)
- 实现 SM-2 间隔重复算法
- 更新所有前端页面
- 添加数据库迁移

src/app/(features)/explore/page.tsx

@@ -1,9 +1,9 @@
 import { ExploreClient } from "./ExploreClient";
-import { actionGetPublicFolders } from "@/modules/folder/folder-action";
+import { actionGetPublicDecks } from "@/modules/deck/deck-action";
 
 export default async function ExplorePage() {
-  const publicFoldersResult = await actionGetPublicFolders();
-  const publicFolders = publicFoldersResult.success ? publicFoldersResult.data ?? [] : [];
+  const publicDecksResult = await actionGetPublicDecks();
+  const publicDecks = publicDecksResult.success ? publicDecksResult.data ?? [] : [];
 
-  return <ExploreClient initialPublicFolders={publicFolders} />;
+  return <ExploreClient initialPublicDecks={publicDecks} />;
 }