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/css/modal.css

@@ -1516,6 +1516,19 @@
     min-width: 0;
 }
 
+.composite-layer-brightness-label {
+    flex-shrink: 0;
+    width: 90px;
+    font-size: 0.8rem;
+    color: var(--text-secondary);
+}
+
+.composite-layer-brightness,
+.composite-layer-cspt {
+    flex: 1;
+    min-width: 0;
+}
+
 .composite-layer-blend {
     width: 100px;
     flex-shrink: 0;