Firebase App Hosting adds instant rollbacks and SDK auto-configuration

Firebase App Hosting now supports instant rollbacks to previous builds without rebuilding, automatic Firebase SDK initialization via the FIREBASE_WEBAPP_CONFIG variable, and VPC network connectivity for backends. Build errors are also surfaced directly in the dashboard.

• Rollback restores an old container image without rebuilding
• Client initializeApp runs with no config object
• VPC access via direct egress and serverless connectors
• Build errors now shown in the App Hosting dashboard