feat: PWA push notifications — service worker, VAPID, subscription management

- Add web-push VAPID backend with push_subscriptions table (migration 078)
- Backend push module with sendPushToUser/sendPushToRoom helpers
- Push routes: GET vapid-key, POST subscribe, DELETE unsubscribe
- Trigger push on new chat messages in direct/group rooms
- PWA manifest, service worker, and icons (72–512px)
- Frontend push lib: SW registration, subscribe/unsubscribe helpers
- Push API in api.ts, SW registered in main.tsx
- usePushSubscription hook for auto-subscribe on permission grant
- PushPermissionRow component in ChatWidget settings view

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/sw.js

@@ -0,0 +1,35 @@
+const CACHE_NAME = 'hotelsync-v1'
+
+self.addEventListener('install', () => self.skipWaiting())
+self.addEventListener('activate', event => event.waitUntil(self.clients.claim()))
+
+self.addEventListener('push', event => {
+  if (!event.data) return
+  let data
+  try { data = event.data.json() } catch { data = { title: 'HotelSync', body: event.data.text() } }
+
+  const title = data.title ?? 'HotelSync'
+  const options = {
+    body: data.body ?? '',
+    icon: data.icon ?? '/icons/icon-192.png',
+    badge: '/icons/icon-72.png',
+    tag: data.tag,
+    renotify: !!data.tag,
+    data: { url: data.url ?? '/' },
+    vibrate: [200, 100, 200],
+  }
+
+  event.waitUntil(self.registration.showNotification(title, options))
+})
+
+self.addEventListener('notificationclick', event => {
+  event.notification.close()
+  const url = event.notification.data?.url ?? '/'
+  event.waitUntil(
+    self.clients.matchAll({ type: 'window', includeUncontrolled: true }).then(clients => {
+      const existing = clients.find(c => c.url.includes(self.location.origin))
+      if (existing) { existing.focus(); existing.navigate(url) }
+      else self.clients.openWindow(url)
+    })
+  )
+})