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/storage/color_strip_source.py

@@ -227,7 +227,7 @@ class ColorStripSource:
             return ApiInputColorStripSource(
                 id=sid, name=name, source_type="api_input",
                 created_at=created_at, updated_at=updated_at, description=description,
-                clock_id=clock_id, tags=tags, led_count=data.get("led_count") or 0,
+                clock_id=clock_id, tags=tags,
                 fallback_color=fallback_color,
                 timeout=float(data.get("timeout") or 5.0),
             )