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

k8s/gitea-assistant.yaml

@@ -56,7 +56,7 @@ spec:
               mountPath: /app/data
           livenessProbe:
             httpGet:
-              path: /
+              path: /api/health
               port: http
             initialDelaySeconds: 10
             periodSeconds: 30
@@ -64,7 +64,7 @@ spec:
             failureThreshold: 3
           readinessProbe:
             httpGet:
-              path: /
+              path: /api/health
               port: http
             initialDelaySeconds: 5
             periodSeconds: 10