Add brightness control to Key Colors targets with HAOS integration

Adds brightness (0.0-1.0) to KeyColorsSettings, applies scaling in the
KC processing pipeline after smoothing, exposes a slider on the WebUI
target card, and creates a HA number entity for KC target brightness.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/storage/key_colors_picture_target.py

@@ -45,6 +45,7 @@ class KeyColorsSettings:
     interpolation_mode: str = "average"
     smoothing: float = 0.3
     pattern_template_id: str = ""
+    brightness: float = 1.0
 
     def to_dict(self) -> dict:
         return {
@@ -52,6 +53,7 @@ class KeyColorsSettings:
             "interpolation_mode": self.interpolation_mode,
             "smoothing": self.smoothing,
             "pattern_template_id": self.pattern_template_id,
+            "brightness": self.brightness,
         }
 
     @classmethod
@@ -61,6 +63,7 @@ class KeyColorsSettings:
             interpolation_mode=data.get("interpolation_mode", "average"),
             smoothing=data.get("smoothing", 0.3),
             pattern_template_id=data.get("pattern_template_id", ""),
+            brightness=data.get("brightness", 1.0),
         )