Google Authentication & Gmail Pub/Sub Watch Setup

[Arkadipttv]

Implemented Google authentication support for Gmail channel setup

Completed

• Added Gmail-specific channel flow.
• Replaced Next with Save for Gmail channel creation.
• Added Gmail channel details UI with connection status.
• Added Connect with Google / Reconnect with Google action.
• Implemented Google OAuth redirect and callback handling.
• Preserved existing WhatsApp channel flow.

Implemented Gmail Pub/Sub watch initialization after Google authentication.

Completed

• Added Gmail API watch setup after successful Connect with Google.
• Auto-saved Pub/Sub topic and Gmail watch response data in channel_metas.
• Stored watch status, history ID, label IDs, expiration, and error details.
• Updated Gmail channel details UI to show Pub/Sub/watch status.
• Added Google API client dependency for Gmail watch support.