Live KC test WS, sync clock fix, device card perf, camera icons, tab indicator

Key Colors test:
- New WS endpoint for live KC target test streaming (replaces REST polling)
- Auto-connect on lightbox open, auto-disconnect on close
- Uses same FPS/preview_width as CSS source test (no separate controls)
- Removed FPS selector, start/stop toggle, and updateAutoRefreshButton

Device cards:
- Fix full re-render on every poll caused by relative "Last seen" time in HTML
- Last seen label now patched in-place via data attribute (like FPS metrics)
- Remove overlay visualization button from LED target cards

Sync clocks:
- Fix card not updating start/stop icon: invalidate cache before reload

Other:
- Tab indicator respects bg-anim toggle (hidden when dynamic background off)
- Camera backend icon grid uses SVG icons instead of emoji
- Frontend context rule: no emoji in IconSelect items

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

server/src/wled_controller/templates/partials/image-lightbox.html

@@ -1,7 +1,13 @@
 <!-- Image Lightbox -->
 <div id="image-lightbox" class="lightbox" onclick="closeLightbox(event)">
     <button class="lightbox-close" onclick="closeLightbox()" title="Close">&#x2715;</button>
-    <button id="lightbox-auto-refresh" class="lightbox-refresh-btn" onclick="toggleKCTestAutoRefresh()" title="Auto-refresh" style="display:none">&#x25B6;</button>
+    <button id="lightbox-auto-refresh" class="lightbox-refresh-btn" onclick="toggleKCTestAutoRefresh()" title="Stream live" style="display:none">&#x25B6;</button>
+    <select id="lightbox-fps-select" class="lightbox-fps-select" onchange="onLightboxFpsChange(this.value)" style="display:none" title="Frames per second">
+        <option value="1">1 fps</option>
+        <option value="2">2 fps</option>
+        <option value="3" selected>3 fps</option>
+        <option value="5">5 fps</option>
+    </select>
     <div class="lightbox-content">
         <img id="lightbox-image" src="" alt="Full size preview">
         <div id="lightbox-stats" class="lightbox-stats" style="display: none;"></div>