Port WLED optimizations to KC loop: fix FPS metrics, add keepalive and auto-refresh test

- Fix KC fps_actual to use frame-to-frame timestamps (was inflated by measuring before sleep)
- Add fps_potential, fps_current, frames_skipped, frames_keepalive metrics to KC loop
- Add keepalive broadcast for static frames so WS clients stay in sync
- Expose all KC metrics in get_kc_target_state() and update UI card to show 7 metrics
- Add auto-refresh play/pause button to KC test lightbox (polls every ~1s)
- Fix WebSocket color swatches computing hex from r,g,b when hex field is absent
- Fix WebSocket auth crash by using get_config() instead of module-level config variable
- Fix lightbox closing when clicking auto-refresh button (event bubbling)

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

server/src/wled_controller/static/index.html

@@ -867,6 +867,7 @@
     <!-- 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>
         <div class="lightbox-content">
             <img id="lightbox-image" src="" alt="Full size preview">
             <div id="lightbox-stats" class="lightbox-stats" style="display: none;"></div>