pwa: 可安装+离线可用（manifest/sw/图标/接线）

- manifest.webmanifest: standalone, start_url /?source=pwa, theme #312E81, 4 icons
- sw.js v1: 预缓存骨架(11项), 导航+静态 network-first, /api/* 与 /mcp/* 不缓存
- icons: 192/512 any + maskable + apple-touch-icon
- 实测: SW activated+controlling, installabilityErrors=[], 离线重载骨架正常渲染

manifest.webmanifest

@@ -0,0 +1,20 @@
+{
+  "name": "工时薪酬助手",
+  "short_name": "工时薪酬",
+  "description": "工时排班记账与多周期薪酬核算，本地优先，离线可用。",
+  "lang": "zh-CN",
+  "dir": "ltr",
+  "id": "/",
+  "start_url": "/?source=pwa",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "portrait",
+  "theme_color": "#312E81",
+  "background_color": "#F8FAFC",
+  "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/icon-maskable-192.png", "sizes": "192x192", "type": "image/png", "purpose": "maskable" },
+    { "src": "/icons/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}