feat: installable PWA (manifest, icons, minimal service worker)

- public/manifest.webmanifest with any + maskable icons (192/512)
- generated inbox-glyph icons in app palette, apple-touch-icon
- minimal no-cache service worker for installability; registered via client plugin
- head: manifest/icon links, mobile-web-app-capable

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/manifest.webmanifest

@@ -0,0 +1,17 @@
+{
+  "id": "/",
+  "name": "ClaudeDo Inbox",
+  "short_name": "ClaudeDo",
+  "description": "Task inbox for the ClaudeDo backlog",
+  "start_url": "/",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#181410",
+  "theme_color": "#181410",
+  "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" }
+  ]
+}