feat: major dashboard overhaul and polish

- Switched from mock data to real backend APIs.
- Implemented unified ApiClient for consistent frontend data fetching.
- Refactored dashboard structure and styles for a modern SaaS aesthetic.
- Fixed Axum 0.8+ routing and parameter syntax issues.
- Implemented real client creation/deletion and provider health monitoring.
- Synchronized WebSocket event structures between backend and frontend.

static/index.html

@@ -159,6 +159,7 @@
     </div>
 
     <!-- Scripts -->
+    <script src="/js/api.js"></script>
     <script src="/js/auth.js"></script>
     <script src="/js/dashboard.js"></script>
     <script src="/js/websocket.js"></script>