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/js/api/probe.js

@@ -0,0 +1,24 @@
+export class ProbeAPI {
+    constructor(baseURL = '') {
+        this.baseURL = baseURL;
+    }
+
+    /**
+     * Probe a device at the given IP address.
+     * Returns device info: reachable status, vendor, hostname, mDNS data.
+     * @param {string} ip - IP address to probe
+     * @returns {Promise<Object>} Probe response
+     */
+    async probe(ip) {
+        const response = await fetch(
+            `${this.baseURL}api/v1/probe?ip=${encodeURIComponent(ip)}`
+        );
+
+        if (!response.ok) {
+            const text = await response.text();
+            throw new Error(text || `HTTP ${response.status}`);
+        }
+
+        return await response.json();
+    }
+}