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>

backend/migrations/078_push_subscriptions.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS push_subscriptions (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  endpoint TEXT NOT NULL UNIQUE,
+  p256dh TEXT NOT NULL,
+  auth TEXT NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_push_subscriptions_user ON push_subscriptions(user_id);