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/sw.js

@@ -0,0 +1,6 @@
+// Minimal service worker: enables PWA installability across browsers.
+// Intentionally no caching — the app is a private, auth-gated SPA and stale
+// assets after a deploy are worse than requiring a connection.
+self.addEventListener("install", () => self.skipWaiting());
+self.addEventListener("activate", (event) => event.waitUntil(self.clients.claim()));
+self.addEventListener("fetch", () => {});