Add host network info display and custom subnet scan to Receivers tab

Shows the server's network interfaces (IP, subnet) in the admin Receivers
tab so users know which networks are being scanned. Adds an optional subnet
input field to scan arbitrary networks (e.g. VLANs the server can route to
but isn't directly attached to), with validation and a /20 size cap.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

admin/admin.js

@@ -27,6 +27,7 @@
         adminPanel.classList.remove('hidden');
         loadConfig();
         loadThemes();
+        loadNetworkInfo();
         refreshStatus();
         statusInterval = setInterval(refreshStatus, 5000);
     }
@@ -170,6 +171,31 @@
         } catch (e) { /* ignore */ }
     }
 
+    // ------- Network Info -------
+    async function loadNetworkInfo() {
+        const el = document.getElementById('network-info');
+        if (!el) return;
+        try {
+            const res = await fetch('/api/admin/network-info');
+            if (!res.ok) { el.textContent = 'Failed to load'; return; }
+            const data = await res.json();
+            const ifaces = data.interfaces || [];
+            if (!ifaces.length) {
+                el.textContent = 'No network interfaces found';
+                return;
+            }
+            let html = '<table class="network-info-table">';
+            html += '<tr><th>Interface</th><th>IP Address</th><th>Network</th></tr>';
+            ifaces.forEach(i => {
+                html += `<tr><td>${i.name}</td><td>${i.ip}</td><td>${i.network}</td></tr>`;
+            });
+            html += '</table>';
+            el.innerHTML = html;
+        } catch (e) {
+            el.textContent = 'Error loading network info';
+        }
+    }
+
     // ------- Receivers -------
     function toggleManualIps(mode) {
         document.getElementById('manual-ips-group').classList.toggle('hidden', mode === 'AUTO');
@@ -184,12 +210,20 @@
         btn.textContent = 'SCANNING...';
         btn.disabled = true;
         try {
-            const res = await fetch('/api/admin/scan-receivers', { method: 'POST' });
+            const subnet = (document.getElementById('scan-subnet').value || '').trim();
+            const fetchOpts = { method: 'POST' };
+            if (subnet) {
+                fetchOpts.headers = { 'Content-Type': 'application/json' };
+                fetchOpts.body = JSON.stringify({ subnet });
+            }
+            const res = await fetch('/api/admin/scan-receivers', fetchOpts);
             const data = await res.json();
             const box = document.getElementById('scan-results');
             const list = document.getElementById('scan-results-list');
             box.classList.remove('hidden');
-            if (data.receivers && data.receivers.length) {
+            if (data.error) {
+                list.textContent = data.error;
+            } else if (data.receivers && data.receivers.length) {
                 list.textContent = data.receivers.join(', ');
             } else {
                 list.textContent = 'No receivers found on network';