Add Web Push notifications, PWA support, and full-screen tablet kiosk

Parents get a real push notification when a kid checks off a task
(false->true transitions only, fire-and-forget, degrades gracefully with
no VAPID keys configured). Dashboard is a fully installable iOS/Android
PWA; each child's kiosk link gets its own dynamic per-token manifest so
"Add to Home Screen" opens straight into their board in standalone mode.

Kiosk view is reworked for tablets: safe-area-aware full-bleed layout,
the whole task row is now tappable (previously only the 24px checkbox
was, well under Apple's touch-target minimum), and app icons are
generated by a small dependency-free PNG encoder (no image tooling
available in this environment).

Push requires real HTTPS (iOS Safari won't allow it otherwise) - README
and UNRAID.md cover VAPID setup and the HTTPS prerequisite.

public/js/api.js

@@ -25,5 +25,5 @@ const api = {
   get(path) { return this.request('GET', path); },
   post(path, body) { return this.request('POST', path, body === undefined ? {} : body); },
   patch(path, body) { return this.request('PATCH', path, body === undefined ? {} : body); },
-  del(path) { return this.request('DELETE', path); },
+  del(path, body) { return this.request('DELETE', path, body); },
 };