feat(pwa): offer to install — Chromium prompt, iOS "Add to Home Screen" hint (#116)

The client never handled beforeinstallprompt and showed no install hint, so
phone users only got the PWA if they knew to dig through Share → Add to
Home Screen (iOS never prompts; Chromium's mini-infobar is easy to miss).

- utils/pwaInstall.ts (pure, 4 tests): show from the second visit, never in
  Tauri or an installed PWA (display-mode standalone / navigator.standalone),
  30-day snooze after a dismissal; kind = real prompt when the browser
  handed us a deferred beforeinstallprompt, Share-sheet instructions on iOS
  Safari, nothing elsewhere (Firefox desktop has no install path).
- hooks/usePwaInstallPrompt.ts: captures beforeinstallprompt/appinstalled,
  counts one visit per browser session, waits 6s for the prompt event before
  deciding, then enqueues a sticky toast; tap → prompt(), X → snooze.
- ToastNotif gains onDismiss (fired by the X button only) so the snooze is
  recorded however the toast is closed. Mounted from ClientNonUIFeatures for
  signed-in users only.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/features/toast/LotusToastContainer.tsx

@@ -63,6 +63,7 @@ function ToastCard({ toast }: ToastCardProps) {
 
   const handleDismiss = (e: React.MouseEvent) => {
     e.stopPropagation();
+    toast.onDismiss?.();
     dismiss(toast.id);
   };