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/locales/zh.json

@@ -979,6 +979,11 @@
   "color_strip.test.fps": "FPS:",
   "color_strip.test.apply": "应用",
   "color_strip.test.composite": "合成",
+  "color_strip.preview.title": "实时预览",
+  "color_strip.preview.not_connected": "未连接",
+  "color_strip.preview.connecting": "连接中...",
+  "color_strip.preview.connected": "已连接",
+  "color_strip.preview.unsupported": "此源类型不支持预览",
   "color_strip.type.daylight": "日光循环",
   "color_strip.type.daylight.desc": "模拟24小时自然日光变化",
   "color_strip.type.daylight.hint": "模拟太阳在24小时内的色温变化——从温暖的日出到冷白的日光，再到温暖的日落和昏暗的夜晚。",