Add FCM push notification support

[dalinaum]

Summary

• Add firebase-messaging dependency and HackersPubMessagingService for receiving FCM data messages
• Implement FcmTokenManager for token registration/unregistration via GraphQL mutations
• Register FCM token on login (via LaunchedEffect(isLoggedIn)), unregister on logout
• Request POST_NOTIFICATIONS permission on login transition (not just app startup)
• Remove foreground polling — FCM replaces it; 15-min WorkManager polling remains as fallback
• Handle all GraphQL union error types (RegisterFcmDeviceTokenFailedError, InvalidInputError, NotAuthenticatedError) in mutation responses
• Add FCM mutation types to local GraphQL schema

⚠️ Requires server-side FCM support: hackers-pub/hackerspub#251 must be merged and deployed first. The GraphQL schema in this PR includes registerFcmDeviceToken / unregisterFcmDeviceToken mutations that the server does not yet serve. After the server PR is deployed, re-run schema introspection to sync.

Test plan

• ./gradlew assembleDebug build passes
• Verify FCM token registration on login (check FcmTokenManager logs)
• Verify push notification received in background after server FCM support is live
• Verify notification permission prompt appears on first login
• Verify FCM token unregistered on logout

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