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/locales/zh.json

@@ -1131,6 +1131,8 @@
   "color_strip.composite.blend_mode.multiply.desc": "通过相乘颜色变暗",
   "color_strip.composite.blend_mode.screen": "滤色",
   "color_strip.composite.blend_mode.screen.desc": "提亮，正片叠底的反转",
+  "color_strip.composite.blend_mode.override": "覆盖",
+  "color_strip.composite.blend_mode.override.desc": "黑色=透明，亮色=不透明",
   "color_strip.composite.opacity": "不透明度",
   "color_strip.composite.brightness": "亮度",
   "color_strip.composite.brightness.none": "无（全亮度）",