Add per-layer brightness source to composite CSS and enhance selectors

- Add optional brightness_source_id per composite layer using ValueStreamManager
- Use EntitySelect for composite layer source and brightness dropdowns
- Use IconSelect for composite blend mode and notification filter mode
- Add i18n keys for blend mode and filter mode descriptions (en/ru/zh)

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

server/src/wled_controller/api/schemas/color_strip_sources.py

@@ -34,6 +34,7 @@ class CompositeLayer(BaseModel):
     blend_mode: str = Field(default="normal", description="Blend mode: normal|add|multiply|screen")
     opacity: float = Field(default=1.0, ge=0.0, le=1.0, description="Layer opacity 0.0-1.0")
     enabled: bool = Field(default=True, description="Whether this layer is active")
+    brightness_source_id: Optional[str] = Field(None, description="Optional value source ID for dynamic brightness")
 
 
 class MappedZone(BaseModel):