feat(pwa): installable app — manifest + SW + Settings install button

The captive-portal Step-2 QR opens pictureframe.edholm.me in Safari,
which is the perfect moment to also offer "pin this to your home
screen" so the recipient gets one-tap access without typing the URL
again. Two pieces:

* Service worker at /sw.js (document root, scope "/"). Minimal —
  install/activate calls skipWaiting + clients.claim, fetch is
  passthrough. Real offline caching is intentionally out of scope;
  we only need the SW to exist so Chrome's PWA-install heuristic
  fires.

* Settings → Install app section, hidden when display-mode standalone.
  Android Chrome path: native beforeinstallprompt button.
  iOS Safari (and any other non-prompt browser): button opens a
  modal with step-by-step Share → Add to Home Screen instructions.

usePwaInstall composable handles the singleton lifecycle —
beforeinstallprompt fires once per page load and may fire before the
user navigates to Settings, so we register on module import and stash
the event for later.

Tests cover: install button rendered when not standalone, modal opens
on click without a native prompt, modal close button works.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/main.ts

@@ -10,3 +10,15 @@ app.use(createPinia())
 app.use(router)
 app.use(VueKonva)
 app.mount('#app')
+
+// Register the PWA service worker. Done after mount so the SW
+// register call doesn't block first paint. /sw.js (not /build/sw.js)
+// so the SW's scope is "/" and covers the whole SPA.
+if ('serviceWorker' in navigator) {
+  window.addEventListener('load', () => {
+    navigator.serviceWorker.register('/sw.js').catch(() => {
+      // SW registration failure is non-fatal — PWA install just won't be
+      // available, but the app still works.
+    })
+  })
+}