Revamp navigation and styling; enhance API URL handling.

Replaced `Link` components with `NavLink` for active state support and added new sidebar navigation styling. Enhanced API URL handling to prevent mixed content when using HTTPS. Updated layout and CSS for better responsiveness and consistent design.

.env.example

@@ -25,4 +25,5 @@ INIT_ADMIN_PASSWORD=ChangeMe123!
 
 # Frontend
 FRONTEND_PORT=5173
-VITE_API_URL=http://localhost:8000/api/v1
+# For reverse proxy + SSL prefer relative path to avoid mixed-content.
+VITE_API_URL=/api/v1