refactor: 移除 Service Worker 离线缓存 + 统一注册引导文案

- 删除 sw.js、ServiceWorkerRegistrar、offline 页面
- 保留 manifest 和 PWA 图标（添加到主屏幕仍可用）
- 注册引导文案统一为"10 秒注册，无需手机号"

public/sw.js

@@ -1,68 +0,0 @@
-const CACHE_NAME = "nowhatever-v1";
-
-const PRECACHE_URLS = ["/", "/offline"];
-
-self.addEventListener("install", (event) => {
-  event.waitUntil(
-    caches
-      .open(CACHE_NAME)
-      .then((cache) => cache.addAll(PRECACHE_URLS))
-      .then(() => self.skipWaiting())
-  );
-});
-
-self.addEventListener("activate", (event) => {
-  event.waitUntil(
-    caches
-      .keys()
-      .then((keys) =>
-        Promise.all(
-          keys
-            .filter((key) => key !== CACHE_NAME)
-            .map((key) => caches.delete(key))
-        )
-      )
-      .then(() => self.clients.claim())
-  );
-});
-
-self.addEventListener("fetch", (event) => {
-  const { request } = event;
-
-  if (request.method !== "GET") return;
-
-  const url = new URL(request.url);
-
-  // API calls: network-only
-  if (url.pathname.startsWith("/api/")) return;
-
-  // Static assets (_next/static, icons, fonts): cache-first
-  if (
-    url.pathname.startsWith("/_next/static/") ||
-    url.pathname.match(/\.(png|jpg|svg|ico|woff2?)$/)
-  ) {
-    event.respondWith(
-      caches.match(request).then(
-        (cached) =>
-          cached ||
-          fetch(request).then((response) => {
-            const clone = response.clone();
-            caches.open(CACHE_NAME).then((cache) => cache.put(request, clone));
-            return response;
-          })
-      )
-    );
-    return;
-  }
-
-  // HTML pages: network-first, fallback to cache, then offline page
-  event.respondWith(
-    fetch(request, { cache: "no-cache" })
-      .then((response) => {
-        const clone = response.clone();
-        caches.open(CACHE_NAME).then((cache) => cache.put(request, clone));
-        return response;
-      })
-      .catch(() => caches.match(request).then((cached) => cached || caches.match("/offline")))
-  );
-});