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/templates/modals/css-editor.html

@@ -621,6 +621,7 @@
         </div>
         <div class="modal-footer">
             <button class="btn btn-icon btn-secondary" onclick="closeCSSEditorModal()" title="Cancel" data-i18n-aria-label="aria.cancel">&#x2715;</button>
+            <button class="btn btn-icon btn-secondary" onclick="previewCSSFromEditor()" data-i18n-title="color_strip.test.title" title="Test Preview"><svg class="icon" viewBox="0 0 24 24"><path d="M14 2v6a2 2 0 0 0 .245.96l5.51 10.08A2 2 0 0 1 18 22H6a2 2 0 0 1-1.755-2.96l5.51-10.08A2 2 0 0 0 10 8V2"/><path d="M6.453 15h11.094"/><path d="M8.5 2h7"/></svg></button>
             <button class="btn btn-icon btn-primary" onclick="saveCSSEditor()" title="Save" data-i18n-aria-label="aria.save">&#x2713;</button>
         </div>
     </div>