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/style.css

@@ -352,6 +352,73 @@ section {
     letter-spacing: 0.5px;
 }
 
+/* Device discovery */
+.discovery-section {
+    margin-bottom: 12px;
+}
+.btn-block {
+    width: 100%;
+}
+.discovery-list {
+    max-height: 200px;
+    overflow-y: auto;
+    display: flex;
+    flex-direction: column;
+    gap: 4px;
+    margin-top: 8px;
+}
+.discovery-item {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 8px 12px;
+    border: 1px solid var(--border-color);
+    border-radius: 4px;
+    background: var(--card-bg);
+    cursor: pointer;
+    transition: opacity 0.15s;
+}
+.discovery-item:not(.discovery-item--added):hover {
+    opacity: 0.8;
+}
+.discovery-item--added {
+    opacity: 0.5;
+    cursor: default;
+}
+.discovery-item-info {
+    display: flex;
+    flex-direction: column;
+    gap: 2px;
+}
+.discovery-badge {
+    font-size: 11px;
+    padding: 2px 8px;
+    border-radius: 10px;
+    background: var(--border-color);
+    white-space: nowrap;
+}
+.modal-divider {
+    border: none;
+    border-top: 1px solid var(--border-color);
+    margin: 12px 0;
+}
+.discovery-loading {
+    display: flex;
+    justify-content: center;
+    padding: 12px;
+}
+.discovery-spinner {
+    width: 20px;
+    height: 20px;
+    border: 2px solid var(--border-color);
+    border-top-color: var(--primary-color);
+    border-radius: 50%;
+    animation: spin 0.8s linear infinite;
+}
+@keyframes spin {
+    to { transform: rotate(360deg); }
+}
+
 .channel-indicator {
     display: inline-flex;
     gap: 2px;
@@ -949,6 +1016,34 @@ input:-webkit-autofill:focus {
     color: var(--text-color);
 }
 
+.modal-header-actions {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+}
+
+.modal-header-btn {
+    background: none;
+    border: none;
+    font-size: 1.1rem;
+    width: 32px;
+    height: 32px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    cursor: pointer;
+    border-radius: 4px;
+    transition: background 0.2s;
+    flex-shrink: 0;
+}
+.modal-header-btn:hover {
+    background: rgba(128, 128, 128, 0.15);
+}
+.modal-header-btn:disabled {
+    opacity: 0.4;
+    cursor: default;
+}
+
 .modal-close-btn {
     background: none;
     border: none;