fix: OAuth2 callback, CORS, and WebSocket connection issues

[Mathieu-bot]

Summary

• Fix OAuth2 login flow by redirecting browser to Spring Security endpoint instead of using POST callback
• Add CORS configuration in SecurityFilterChain to allow frontend origins (localhost:5173) with credentials
• Allow /ws/** endpoints for WebSocket handshake
• Redirect to frontend after OAuth2 success and logout

Changes

1. SecurityConfig.java: Added CORS source bean, allowed /ws/**, configured OAuth2 success/logout redirects to frontend
2. application.yml: Added localhost:5173 to default CORS origins
3. CorsConfigurer.java: Updated default origins to include localhost:5173

Testing

• OAuth2 Google login now redirects correctly to frontend after authentication
• CORS headers properly set for API requests from localhost:5173
• WebSocket STOMP connections work with SockJS fallback

[Mathieu-bot]

Ok