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/decks/page.tsx

@@ -0,0 +1,16 @@
+import { auth } from "@/auth";
+import { DecksClient } from "./DecksClient";
+import { headers } from "next/headers";
+import { redirect } from "next/navigation";
+
+export default async function DecksPage() {
+  const session = await auth.api.getSession(
+    { headers: await headers() }
+  );
+
+  if (!session) {
+    redirect("/login?redirect=/decks");
+  }
+
+  return <DecksClient userId={session.user.id} />;
+}