Add WLED auto-discovery via mDNS with zeroconf

Scan the local network for WLED devices advertising _wled._tcp.local.
and present them in the Add Device modal for one-click selection.

- New discovery.py: async mDNS browse + parallel /json/info enrichment
- GET /api/v1/devices/discover endpoint with already_added dedup
- Header scan button (magnifying glass icon) in add-device modal
- Discovered devices show name, IP, LED count, version; click to fill form
- en/ru locale strings for discovery UI

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

server/src/wled_controller/static/index.html

@@ -570,9 +570,22 @@
         <div class="modal-content">
             <div class="modal-header">
                 <h2 data-i18n="devices.add">Add New Device</h2>
-                <button class="modal-close-btn" onclick="closeAddDeviceModal()" title="Close">&#x2715;</button>
+                <div class="modal-header-actions">
+                    <button type="button" class="modal-header-btn" id="scan-network-btn" onclick="scanForDevices()" data-i18n-title="device.scan" title="Auto Discovery">&#x1F50D;</button>
+                    <button class="modal-close-btn" onclick="closeAddDeviceModal()" title="Close">&#x2715;</button>
+                </div>
             </div>
             <div class="modal-body">
+                <div id="discovery-section" class="discovery-section" style="display: none;">
+                    <div id="discovery-loading" class="discovery-loading" style="display: none;">
+                        <span class="discovery-spinner"></span>
+                    </div>
+                    <div id="discovery-list" class="discovery-list"></div>
+                    <div id="discovery-empty" style="display: none;">
+                        <small data-i18n="device.scan.empty">No WLED devices found on the network</small>
+                    </div>
+                    <hr class="modal-divider">
+                </div>
                 <form id="add-device-form">
                     <div class="form-group">
                         <div class="label-row">