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>

.gitignore

@@ -9,4 +9,5 @@ node_modules/
 .DS_Store
 .playwright-mcp/
 *.png
+!public/icons/*.png
 .claude/