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/sw.js

@@ -0,0 +1,41 @@
+self.addEventListener('install', () => {
+  self.skipWaiting();
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(self.clients.claim());
+});
+
+self.addEventListener('push', (event) => {
+  if (!event.data) return;
+  const payload = event.data.json();
+  event.waitUntil(
+    self.registration.showNotification(payload.title, {
+      body: payload.body,
+      icon: '/icons/icon-192.png',
+      badge: '/icons/icon-192.png',
+      tag: 'kc-task-done',
+      data: { url: payload.url || '/dashboard.html' },
+    })
+  );
+});
+
+self.addEventListener('notificationclick', (event) => {
+  event.notification.close();
+  const targetUrl = event.notification.data && event.notification.data.url;
+  if (!targetUrl) return;
+
+  event.waitUntil(
+    self.clients.matchAll({ type: 'window', includeUncontrolled: true }).then((clients) => {
+      const existing = clients.find((c) => {
+        try {
+          return new URL(c.url).pathname === new URL(targetUrl, self.location.origin).pathname;
+        } catch {
+          return false;
+        }
+      });
+      if (existing) return existing.focus();
+      return self.clients.openWindow(targetUrl);
+    })
+  );
+});