feat: Add account-level SSO configuration

This implements Single Sign-On (SSO) functionality at the account level, allowing individual accounts to configure their own SSO settings.

Key changes:
- Added SSO configuration to account model with JSONB storage
- Implemented SSO settings UI in account settings panel
- Added feature flag to control SSO access (premium feature)
- Updated authentication flow to support account-level SSO
- Added proper validation and error handling

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/javascript/dashboard/featureFlags.js

@@ -39,6 +39,7 @@ export const FEATURE_FLAGS = {
   CONTACT_CHATWOOT_SUPPORT_TEAM: 'contact_chatwoot_support_team',
   WHATSAPP_EMBEDDED_SIGNUP: 'whatsapp_embedded_signup',
   CAPTAIN_V2: 'captain_integration_v2',
+  SSO: 'sso',
 };
 
 export const PREMIUM_FEATURES = [
@@ -48,4 +49,5 @@ export const PREMIUM_FEATURES = [
   FEATURE_FLAGS.AUDIT_LOGS,
   FEATURE_FLAGS.HELP_CENTER,
   FEATURE_FLAGS.CAPTAIN_V2,
+  FEATURE_FLAGS.SSO,
 ];