Rework API input CSS: segments, remove led_count, HAOS light, test preview

API Input CSS rework:
- Remove led_count field from ApiInputColorStripSource (always auto-sizes)
- Add segment-based payload: solid, per_pixel, gradient modes
- Segments applied in order (last wins on overlap), auto-grow buffer
- Backward compatible: legacy {"colors": [...]} still works
- Pydantic validation: mode-specific field requirements

Test preview:
- Enable test preview button on api_input cards
- Hide LED/FPS controls for api_input (sender controls those)
- Show input source selector for all CSS tests (preselected)
- FPS sparkline chart using shared createFpsSparkline (same as target cards)
- Server only sends frames when push_generation changes (no idle frames)

HAOS integration:
- New light.py: ApiInputLight entity per api_input source (RGB + brightness)
- turn_on pushes solid segment, turn_off pushes fallback color
- Register wled_screen_controller.set_leds service for arbitrary segments
- New services.yaml with field definitions
- Coordinator: push_colors() and push_segments() methods
- Platform.LIGHT added to platforms list

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

custom_components/wled_screen_controller/strings.json

@@ -31,6 +31,11 @@
         "name": "{scene_name}"
       }
     },
+    "light": {
+      "api_input_light": {
+        "name": "Light"
+      }
+    },
     "switch": {
       "processing": {
         "name": "Processing"
@@ -66,5 +71,21 @@
         "name": "Brightness Source"
       }
     }
+  },
+  "services": {
+    "set_leds": {
+      "name": "Set LEDs",
+      "description": "Push segment data to an api_input color strip source.",
+      "fields": {
+        "source_id": {
+          "name": "Source ID",
+          "description": "The api_input CSS source ID (e.g., css_abc12345)."
+        },
+        "segments": {
+          "name": "Segments",
+          "description": "List of segment objects with start, length, mode, and color/colors fields."
+        }
+      }
+    }
   }
 }