Add installable PWA support

public/manifest.webmanifest

@@ -0,0 +1,12 @@
+{
+  "name": "大学英语六级 · 90天打卡",
+  "short_name": "CET-6 打卡",
+  "description": "CET-6 90 天复习计划、云端打卡与复盘。",
+  "lang": "zh-CN",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#eef4ff",
+  "theme_color": "#1d4ed8",
+  "icons": [{ "src": "/favicon.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any maskable" }]
+}

public/sw.js

@@ -0,0 +1,10 @@
+const CACHE = "cet6-shell-v1";
+const APP_SHELL = ["/", "/tasks", "/stats", "/review", "/manifest.webmanifest", "/favicon.svg"];
+self.addEventListener("install", event => event.waitUntil(caches.open(CACHE).then(cache => cache.addAll(APP_SHELL)).then(() => self.skipWaiting())));
+self.addEventListener("activate", event => event.waitUntil(self.clients.claim()));
+self.addEventListener("fetch", event => {
+  const request = event.request;
+  if (request.method !== "GET" || new URL(request.url).origin !== location.origin) return;
+  if (request.url.includes("/api/")) return;
+  event.respondWith(fetch(request).then(response => { const copy = response.clone(); caches.open(CACHE).then(cache => cache.put(request, copy)); return response; }).catch(() => caches.match(request).then(found => found || caches.match("/"))));
+});