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.html

@@ -76,6 +76,10 @@
             <!-- Receivers Tab -->
             <section id="tab-receivers" class="tab-pane hidden">
                 <h2>Receiver Configuration</h2>
+                <div id="network-info-box" class="info-box">
+                    <h3>Host Network</h3>
+                    <div id="network-info">Loading...</div>
+                </div>
                 <div class="form-group">
                     <label>Discovery Mode</label>
                     <select id="receiver-mode">
@@ -91,6 +95,10 @@
                     <label>Receiver Port</label>
                     <input type="number" id="receiver-port" value="30003" min="1" max="65535">
                 </div>
+                <div class="form-group">
+                    <label>Custom Subnet (optional)</label>
+                    <input type="text" id="scan-subnet" placeholder="e.g. 10.0.0.0/24 (leave empty to scan all local subnets)">
+                </div>
                 <div class="button-row">
                     <button id="scan-receivers-btn" class="btn">SCAN NOW</button>
                     <button id="restart-receivers-btn" class="btn">RESTART CONNECTIONS</button>