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.css

@@ -325,6 +325,35 @@ textarea {
     margin-bottom: 10px;
 }
 
+#network-info-box {
+    margin-top: 0;
+    margin-bottom: 16px;
+}
+
+.network-info-table {
+    width: 100%;
+    border-collapse: collapse;
+    font-size: 12px;
+}
+
+.network-info-table th,
+.network-info-table td {
+    text-align: left;
+    padding: 6px 10px;
+    border-bottom: 1px solid rgba(92, 148, 252, 0.3);
+}
+
+.network-info-table th {
+    font-family: 'Press Start 2P', monospace;
+    font-size: 8px;
+    color: #b4d4ec;
+}
+
+.network-info-table td {
+    color: #54fc54;
+    font-family: 'Courier New', monospace;
+}
+
 /* Toast Notification */
 .toast {
     position: fixed;