Override blend mode, FPS sparkline, fix api_input persistence

New features:
- Override composite blend mode: per-pixel alpha from brightness
  (black=transparent, bright=opaque). Ideal for API input over effects.
- API input test preview FPS chart uses shared createFpsSparkline
  (same look as target card charts)

Fixes:
- Fix api_input source not surviving server restart: from_dict was
  still passing removed led_count field to constructor
- Fix composite layer brightness/processing selectors not aligned:
  labels get fixed width, selects fill remaining space
- Fix CSPT input selector showing in non-CSPT CSS test mode
- Fix test modal LED/FPS controls showing for api_input sources
- Server only sends test WS frames when api_input push_generation changes

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

server/src/wled_controller/static/js/features/color-strips.js

@@ -685,6 +685,7 @@ function _getCompositeBlendItems() {
         { value: 'add',      icon: _icon(P.sun),      label: t('color_strip.composite.blend_mode.add'),      desc: t('color_strip.composite.blend_mode.add.desc') },
         { value: 'multiply', icon: _icon(P.eye),      label: t('color_strip.composite.blend_mode.multiply'), desc: t('color_strip.composite.blend_mode.multiply.desc') },
         { value: 'screen',   icon: _icon(P.monitor),  label: t('color_strip.composite.blend_mode.screen'),   desc: t('color_strip.composite.blend_mode.screen.desc') },
+        { value: 'override', icon: _icon(P.zap),      label: t('color_strip.composite.blend_mode.override'), desc: t('color_strip.composite.blend_mode.override.desc') },
     ];
 }
 
@@ -741,6 +742,7 @@ function _compositeRenderList() {
                         <option value="add"${layer.blend_mode === 'add' ? ' selected' : ''}>${t('color_strip.composite.blend_mode.add')}</option>
                         <option value="multiply"${layer.blend_mode === 'multiply' ? ' selected' : ''}>${t('color_strip.composite.blend_mode.multiply')}</option>
                         <option value="screen"${layer.blend_mode === 'screen' ? ' selected' : ''}>${t('color_strip.composite.blend_mode.screen')}</option>
+                        <option value="override"${layer.blend_mode === 'override' ? ' selected' : ''}>${t('color_strip.composite.blend_mode.override')}</option>
                     </select>
                 </div>
                 <div class="composite-layer-row">