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" }]
+}