refactor(pwa): migrate from @ducanh2912/next-pwa to @serwist/turbopack

Next.js 16 ships Turbopack as the default bundler, but
@ducanh2912/next-pwa is webpack-only — the previous PWA setup
required `--webpack` on dev/build/"dev:proxy" scripts, accepting
a perf regression in exchange for working PWA install.

@serwist/turbopack (paired with the official 'serwist' package
and 'esbuild') is the Turbopack-native successor: it works with
the default bundler, no opt-out flag, and gives us a real TS
service worker source file we can read and edit.

What this commit does:

1. Swap deps in package.json:
   - Remove @ducanh2912/next-pwa
   - Add @serwist/turbopack, serwist, esbuild (all devDeps)
   - Drop --webpack flag from dev / dev:proxy / build scripts
     (Turbopack is restored as the bundler)

2. next.config.ts:
   - Replace withPWAInit wrapping with withSerwist named import
   - Update comment block to describe new architecture

3. New src/app/sw.ts: Serwist service worker source.
   skipWaiting + clientsClaim + navigationPreload + defaultCache.
   This file is compiled to a real SW at build time by Serwist.

4. New src/app/serwist/[[...slug]]/route.ts: catch-all Route
   Handler that serves the compiled SW at /serwist/sw.js plus
   Serwist's chunked runtime assets. Catch-all is required
   because Serwist serves multiple files under /serwist/.

5. src/app/layout.tsx: replace <SwRegister /> with
   <SerwistProvider swUrl="/serwist/sw.js"> from
   @serwist/turbopack/react. The provider handles registration
   + lifecycle internally.

6. Delete src/app/_components/core/sw-register.tsx: replaced
   by SerwistProvider.

7. .gitignore: add public/sw.js and public/workbox-*.js
   (defensive — these are build artifacts that change hash
   every build and shouldn't be committed).

8. Untrack public/workbox-3c9d0171.js: stale workbox runtime
   from the previous next-pwa build, hash is build-specific
   so it can never be regenerated correctly.

Not modified (intentionally):
- src/utils/pwa.ts: pure beforeinstallprompt browser-API wrapper,
  library-agnostic, stays as-is.
- pwa-install-overlay / pwa-install-dialog: install UI flow
  unchanged.
- public/manifest.json: still works; layout's metadata.manifest
  points at it.

Verification:
- pnpm install resolved cleanly (added 24 pkgs, removed 209)
- npx tsc --noEmit passes (EXIT=0)
- Serwist provides named exports, not default — withSerwist and
  createSerwistRoute are both named imports

src/app/_components/core/sw-register.tsx

@@ -1,34 +0,0 @@
-"use client";
-/**
- * Service Worker 注册器
- *
- * 背景：`@ducanh2912/next-pwa` 配置 `register: false`，不自动注入注册脚本。
- * App Router 下手动注册更可控 —— 不污染 SSR / hydration，可显式做生产 / 开发分流。
- *
- * 挂载位置：layout body 末尾，全站生效。
- *
- * 行为：
- *   - dev (`NODE_ENV === "development"`)：跳过注册
- *     1. next-pwa `disable: true` 时 dev 不生成 sw.js，注册会 404
- *     2. dev HMR + 老 SW cache 互相干扰，开发体验差
- *   - 生产：调 `navigator.serviceWorker.register("/sw.js", { scope: "/" })`
- *     - `scope: "/"` 与 manifest.scope 一致
- *     - `updateViaCache: "none"` 强制每次都拉新版 SW（next-pwa 默认就是这行为，显式更清晰）
- */
-import { useEffect } from "react";
-
-export function SwRegister() {
-  useEffect(() => {
-    if (typeof window === "undefined") return;
-    if (!("serviceWorker" in navigator)) return;
-    if (process.env.NODE_ENV !== "production") return;
-
-    void navigator.serviceWorker.register("/sw.js", {
-      scope: "/",
-      updateViaCache: "none",
-    });
-  }, []);
-
-  // 无 UI —— 纯副作用组件
-  return null;
-}