Add transient preview WS endpoint and test button in CSS editor modal

- Add /color-strip-sources/preview/ws endpoint for ad-hoc source preview
  without saving (accepts full config JSON, streams RGB frames)
- Add test preview button (flask icon) to CSS editor modal footer
- For self-contained types (static, gradient, color_cycle, effect, daylight,
  candlelight), always previews current form values via transient WS
- For non-previewable types, falls back to saved source test endpoint
- Fix route ordering: preview/ws registered before {source_id}/ws
- Fix css-test-led-control label alignment (display: inline globally)
- Add gradient onChange callback for future live-update support
- Add i18n keys for preview (en/ru/zh)

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

server/src/wled_controller/static/js/features/css-gradient-editor.js

@@ -29,6 +29,10 @@ export function hexToRgbArray(hex) {
 let _gradientStops = [];
 let _gradientSelectedIdx = -1;
 let _gradientDragging = null;  // { idx, trackRect } while dragging
+let _gradientOnChange = null;
+
+/** Set a callback that fires whenever stops change. */
+export function gradientSetOnChange(fn) { _gradientOnChange = fn; }
 
 /** Read-only accessor for save/dirty-check from the parent module. */
 export function getGradientStops() {
@@ -183,6 +187,7 @@ export function gradientRenderAll() {
     _gradientRenderCanvas();
     _gradientRenderMarkers();
     _gradientRenderStopList();
+    if (_gradientOnChange) _gradientOnChange();
 }
 
 function _gradientRenderCanvas() {