feat(realtime): service worker support and FCM fallback for device tokens

[3m1n3nc3]

Summary

Makes RealtimeClient.deviceToken() usable for browser push in real apps.

• Service worker option: deviceToken({ serviceWorker }) accepts a ServiceWorkerRegistration or a script URL. A URL is registered with the Firebase web config passed as the firebaseConfig query parameter, so a static worker file can initialize Firebase with no build step or secrets. Without the option, Firebase's default /firebase-messaging-sw.js lookup still applies.
• Unsupported browsers: returns null where web push is unavailable (via isSupported()) instead of throwing.
• FCM fallback: when the streaming transport can't issue tokens, deviceToken() uses the client's firebase config. This covers notifications streamed over Pusher or polling and pushed over FCM; the server's Broadcast.clientConfig() already includes that config when drivers.realtime.push is 'firebase'.
• New helper: requestDeviceToken(firebaseConfig, options) issues a token without a realtime client.
• Docs: the notifications guide has a "Registering a browser for push" section with a ready-to-use push-sw.js. It shows the notification, honors retractions, and opens the notification's link on click.

Test plan

• New tests:
  • registers by URL, passing the config through the query string;
  • passes an existing registration through;
  • returns null where push is unsupported;
  • falls back to the firebase config;
  • returns null when nothing can issue a token.
• pnpm lint, pnpm build, pnpm docs:build, realtime tsc --noEmit
• vitest run: 576 passed, 5 skipped
• Manual check in a browser against a real Firebase project

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
packages/realtime/src/push.ts	93.10%	2 Missing ⚠️

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