Backend:
- Added GET /api/v1/system/credentials to list configured services
- Added DELETE /api/v1/system/credentials/{service}
- Updated CredentialsService to support listing without revealing secrets
Frontend:
- Updated Settings page with Tabs
- Added 'Integrations' tab to manage credentials
- Allows Configuring (Update) and Deleting credentials
- Lists status of Appwrite, Stripe, Firebase, etc.
