Add site-grouped device list in sidebar

Fetch device sites from /api/v1/deviceSites in parallel with devices,
then group cameras by site with collapsible headers. Search now also
matches site names. Falls back to flat list if sites API is unavailable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

styles.css

@@ -167,6 +167,53 @@ body {
     box-shadow: 0 0 6px rgba(244, 67, 54, 0.6);
 }
 
+/* Site Group Headers */
+.site-group-header {
+    display: flex;
+    align-items: center;
+    gap: 6px;
+    padding: 8px 10px;
+    margin-bottom: 4px;
+    cursor: pointer;
+    border-radius: 4px;
+    user-select: none;
+    transition: background 0.15s ease;
+}
+
+.site-group-header:hover {
+    background: var(--hover-bg);
+}
+
+.site-group-arrow {
+    font-size: 10px;
+    color: var(--text-secondary);
+    width: 12px;
+    flex-shrink: 0;
+    text-align: center;
+}
+
+.site-group-name {
+    font-size: 11px;
+    font-weight: bold;
+    color: var(--accent-primary);
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+    flex: 1;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
+.site-group-count {
+    font-size: 10px;
+    color: var(--text-secondary);
+    background: var(--bg-primary);
+    border: 1px solid var(--border);
+    border-radius: 10px;
+    padding: 1px 6px;
+    flex-shrink: 0;
+}
+
 .placeholder-text {
     color: var(--text-secondary);
     font-style: italic;