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>

contexts/frontend.md

@@ -78,6 +78,8 @@ Plain `<select>` dropdowns should be enhanced with visual selectors depending on
 
 Both widgets hide the native `<select>` but keep it in the DOM with its value in sync. After programmatically changing the `<select>` value, call `.refresh()` (EntitySelect) or `.setValue(val)` (IconSelect) to update the trigger display. Call `.destroy()` when the modal closes.
 
+**IMPORTANT:** For `IconSelect` item icons, use SVG icons from `js/core/icon-paths.js` (via `_icon(P.iconName)`) or styled `<span>` elements (e.g., `<span style="font-weight:bold">A</span>`). **Never use emoji** — they render inconsistently across platforms and themes.
+
 ### Modal dirty check (discard unsaved changes)
 
 Every editor modal **must** have a dirty check so closing with unsaved changes shows a "Discard unsaved changes?" confirmation. Use the `Modal` base class pattern from `js/core/modal.js`: