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/css/modal.css

@@ -342,6 +342,11 @@
     font-size: 0.85em;
 }
 
+.css-test-led-control label {
+    display: inline;
+    margin-bottom: 0;
+}
+
 .css-test-led-input {
     width: 70px;
     flex: 0 0 70px;
@@ -1263,3 +1268,4 @@
     height: 26px;
     flex: 0 0 26px;
 }
+