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/manifest.webmanifest

@@ -0,0 +1,14 @@
+{
+  "name": "Kids Calendar",
+  "short_name": "Calendar",
+  "start_url": "/dashboard.html",
+  "scope": "/",
+  "display": "standalone",
+  "background_color": "#F1F5FB",
+  "theme_color": "#4A90D9",
+  "icons": [
+    { "src": "/icons/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icons/icon-512.png", "sizes": "512x512", "type": "image/png" },
+    { "src": "/icons/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
+  ]
+}