fix(frontend): standardize favicon/title, 401 redirect, SPA root route, and theme switching

- Replace default Vite favicon and title with project-specific branding
- Add axios response interceptor to handle 401 by clearing token and redirecting to login
- Move health check endpoint from '/' to '/api/health' so SPA index.html is served on root
- Integrate next-themes ThemeProvider with system preference detection and manual toggle
- Update docker-compose and k8s health check paths accordingly
- Replace hardcoded dark-only colors with semantic CSS variable tokens for theme compatibility

docker-compose.yml

@@ -19,7 +19,7 @@ services:
     restart: unless-stopped
 
     healthcheck:
-      test: ["CMD", "curl", "-f", "http://localhost:3000/"]
+      test: ["CMD", "curl", "-f", "http://localhost:3000/api/health"]
       interval: 30s
       timeout: 5s
       retries: 3