Replace deprecated FCM server key authentication with Google service
account OAuth2 flow. The class now creates a signed JWT from the
service account credentials, exchanges it for a short-lived access
token (cached via WordPress transients), and sends messages to the
FCM v2 endpoint (projects/{id}/messages:send).
Settings page updated: FCM Server Key field replaced with Firebase
Project ID + Service Account JSON textarea with validation.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Backend: Add /voice/token endpoint with AccessToken + VoiceGrant for
mobile VoIP, implement unhold_call() with call leg detection, wire FCM
push notifications into call queue and webhook missed call handlers,
add data-only FCM message support for Android background wake, and add
Twilio API Key / Push Credential settings fields.
Flutter app: Full softphone with Twilio Voice SDK integration, JWT auth
with auto-refresh, SSE real-time queue updates, FCM push notifications,
Material 3 UI with dashboard, active call screen, dialpad, and call
controls (mute/speaker/hold/transfer).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>