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>

app/plugins/pwa.client.ts

@@ -0,0 +1,6 @@
+export default defineNuxtPlugin(() => {
+  if (!("serviceWorker" in navigator)) return;
+  window.addEventListener("load", () => {
+    navigator.serviceWorker.register("/sw.js").catch(() => {});
+  });
+});