Add minimal installable PWA support

- serve manifest and service worker from the app root for install compatibility
- add manifest metadata, service worker registration, and Apple touch icon links to the base template
- add install icon assets for Android, iOS, and desktop install flows
- document deployment and validation notes for the new PWA support
- cover the new endpoints and template output with tests

app/static/manifest.webmanifest

@@ -0,0 +1,31 @@
+{
+  "name": "搬家助手",
+  "short_name": "搬家助手",
+  "description": "用于记录搬家装箱内容并快速搜索的轻量工具。",
+  "start_url": "/boxes",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait-primary",
+  "background_color": "#f4f4f4",
+  "theme_color": "#0b57d0",
+  "icons": [
+    {
+      "src": "/static/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/static/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/static/icons/icon-512-maskable.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}

app/static/service-worker.js

@@ -0,0 +1,7 @@
+self.addEventListener("install", function () {
+    self.skipWaiting();
+});
+
+self.addEventListener("activate", function (event) {
+    event.waitUntil(self.clients.claim());
+});