Integrate probe endpoint into frontend

- Add ProbeAPI client (js/api/probe.js)
- Add reusable modal component (js/ui/modal.js) with overlay, animations
- Call GET /api/v1/probe after Check Address click
- Auto-fill Camera Model with vendor from ARP/OUI lookup
- Show modal on unreachable device with Change IP / Continue Anyway buttons
- Add modal CSS styles matching existing dark theme

webui/web/index.html

@@ -643,6 +643,9 @@
         </div>
     </div>
 
+    <!-- Modal -->
+    <div id="modal-overlay" class="modal-overlay hidden"></div>
+
     <!-- Toast Notification -->
     <div id="toast" class="toast hidden"></div>