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.

package.json

@@ -14,6 +14,7 @@
     "bcryptjs": "^2.4.3",
     "express": "^4.19.2",
     "express-rate-limit": "^7.4.0",
-    "express-session": "^1.18.0"
+    "express-session": "^1.18.0",
+    "web-push": "^3.6.7"
   }
 }