Add OAuth integration for multiple providers and implement user linking

- Created OAuth credentials setup guide for Google, Microsoft, Keycloak, ORY, and ZITADEL.
- Added migration for oauth_identity table to store OAuth identities linked to users.
- Implemented OAuthIdentity model for managing OAuth identities in the database.
- Developed oauthService to handle OAuth login, user creation, and identity linking.
- Created OAuthCallbackView and OAuthUserCallbackView components for handling OAuth responses in the frontend.
- Added error handling and user feedback during the OAuth process.

backend/routers/authRouter.js

@@ -9,8 +9,15 @@ router.post('/register', authController.register);
 router.post('/login', authController.login);
 router.post('/forgot-password', authController.forgotPassword);
 router.post('/activate', authController.activateAccount);
+router.get('/oauth/providers', authController.oauthProviders);
+router.get('/oauth/:provider/start', authController.oauthStart);
+router.post('/oauth/exchange', authController.oauthExchange);
 
 // Geschützte Routen (Authentifizierung erforderlich)
 router.get('/logout', authController.logout);
+router.get('/oauth/user/identities', authController.oauthUserIdentities);
+router.get('/oauth/user/:provider/start', authController.oauthUserStart);
+router.post('/oauth/user/exchange', authController.oauthUserExchange);
+router.delete('/oauth/user/:identityId', authController.oauthUserRemove);
 
 export default router;