Add installable PWA shell with safe updates and offline status

web/public/icon.svg

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+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512">
+  <rect width="512" height="512" rx="104" fill="#213547"/>
+  <path d="M142 362V150h112c98 0 113 130 17 145l83 67h-85l-67-65v65zm60-119h49c38 0 38-40 0-40h-49z" fill="#42d392"/>
+</svg>

web/public/manifest.webmanifest

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+{
+  "id": "/",
+  "name": "RawMark",
+  "short_name": "RawMark",
+  "lang": "zh-CN",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#ffffff",
+  "theme_color": "#ffffff",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icons/maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}