feat(ui): add PWA configuration with service worker and enhanced manifest

Adds service worker with network-first navigation (SPA fallback) and
cache-first static assets. Enhances web manifest with start_url, scope,
id, description, orientation, and maskable icon entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/public/site.webmanifest

@@ -1,6 +1,14 @@
 {
+  "id": "/",
   "name": "Paperclip",
   "short_name": "Paperclip",
+  "description": "AI-powered project management and agent coordination platform",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "orientation": "any",
+  "theme_color": "#18181b",
+  "background_color": "#18181b",
   "icons": [
     {
       "src": "/android-chrome-192x192.png",
@@ -11,9 +19,12 @@
       "src": "/android-chrome-512x512.png",
       "sizes": "512x512",
       "type": "image/png"
+    },
+    {
+      "src": "/android-chrome-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     }
-  ],
-  "theme_color": "#18181b",
-  "background_color": "#18181b",
-  "display": "standalone"
+  ]
 }