feat: implement OAuth 2.0 / OpenID Connect support

- Added OAuth configuration management in the admin panel.
- Implemented OAuth authorization flow with PKCE for enhanced security.
- Created routes for handling OAuth provider discovery, authorization, and callback.
- Integrated OAuth login and registration options in the frontend.
- Updated UI components to support OAuth login and registration.
- Added internationalization strings for OAuth-related messages.
- Implemented encryption for client secrets and secure state management.
- Added error handling and user feedback for OAuth processes.

src/pages/Register.jsx

@@ -3,7 +3,7 @@ import { Link, useNavigate, useSearchParams } from 'react-router-dom';
 import { useAuth } from '../contexts/AuthContext';
 import { useLanguage } from '../contexts/LanguageContext';
 import { useBranding } from '../contexts/BrandingContext';
-import { Mail, Lock, User, ArrowRight, Loader2, CheckCircle, ShieldAlert } from 'lucide-react';
+import { Mail, Lock, User, ArrowRight, Loader2, CheckCircle, ShieldAlert, LogIn } from 'lucide-react';
 import BrandLogo from '../components/BrandLogo';
 import toast from 'react-hot-toast';
 
@@ -19,7 +19,7 @@ export default function Register() {
   const [needsVerification, setNeedsVerification] = useState(false);
   const { register } = useAuth();
   const { t } = useLanguage();
-  const { registrationMode } = useBranding();
+  const { registrationMode, oauthEnabled, oauthDisplayName } = useBranding();
   const navigate = useNavigate();
 
   // Invite-only mode without a token → show blocked message
@@ -197,6 +197,26 @@ export default function Register() {
             </button>
           </form>
 
+          {oauthEnabled && (
+            <>
+              <div className="relative my-6">
+                <div className="absolute inset-0 flex items-center">
+                  <div className="w-full border-t border-th-border" />
+                </div>
+                <div className="relative flex justify-center text-xs">
+                  <span className="px-3 bg-th-card/80 text-th-text-s">{t('auth.orContinueWith')}</span>
+                </div>
+              </div>
+              <a
+                href="/api/oauth/authorize"
+                className="btn-secondary w-full py-3 flex items-center justify-center gap-2"
+              >
+                <LogIn size={18} />
+                {t('auth.registerWithOAuth').replace('{provider}', oauthDisplayName || 'SSO')}
+              </a>
+            </>
+          )}
+
           <p className="mt-6 text-center text-sm text-th-text-s">
             {t('auth.hasAccount')}{' '}
             <Link to="/login" className="text-th-accent hover:underline font-medium">