feat: 重构博客为水墨纸质风格 + 搭建后台管理系统

- 重新设计全站 UI：parchment/ink/terracotta 水墨纸质色系，宋式 serif 排版
- 新增页面：文章列表、文章详情、分类、标签、关于
- GSAP ScrollTrigger 滚动动画 + 逐字揭示效果
- 后台管理系统 /admin：文章/分类/标签 CRUD，JSON 文件存储
- 登录认证（cookie session）
- 设计系统文档 UI.md

src/app/api/auth/route.ts

@@ -0,0 +1,36 @@
+import { NextRequest, NextResponse } from "next/server";
+import { cookies } from "next/headers";
+
+const ADMIN_PASSWORD = "asui2026"; // 后续可改环境变量
+const SESSION_KEY = "admin_session";
+const SESSION_VALUE = "authenticated";
+
+export async function POST(request: NextRequest) {
+  const body = await request.json();
+
+  if (body.password === ADMIN_PASSWORD) {
+    const cookieStore = await cookies();
+    cookieStore.set(SESSION_KEY, SESSION_VALUE, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "lax",
+      maxAge: 60 * 60 * 24 * 7, // 7 days
+      path: "/",
+    });
+    return NextResponse.json({ ok: true });
+  }
+
+  return NextResponse.json({ error: "密码错误" }, { status: 401 });
+}
+
+export async function DELETE() {
+  const cookieStore = await cookies();
+  cookieStore.delete(SESSION_KEY);
+  return NextResponse.json({ ok: true });
+}
+
+export async function GET() {
+  const cookieStore = await cookies();
+  const session = cookieStore.get(SESSION_KEY);
+  return NextResponse.json({ authenticated: session?.value === SESSION_VALUE });
+}