feat: add per-layer LED range and collapsible layers to composite source

Composite layers now support optional start/end LED range (toggleable)
and reverse flag, making composite a superset of mapped source. Layers
are collapsible with animated expand/collapse and consistent 0.85rem
font sizing. Delete button restyled as ghost icon button.

Also includes minor dashboard CSS overflow fixes.

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

@@ -36,6 +36,9 @@ class CompositeLayer(BaseModel):
     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")
     processing_template_id: Optional[str] = Field(None, description="Optional color strip processing template ID")
+    start: int = Field(default=0, ge=0, description="First LED index for range (0 = full strip)")
+    end: int = Field(default=0, ge=0, description="Last LED index exclusive for range (0 = full strip)")
+    reverse: bool = Field(default=False, description="Reverse layer output within its range")
 
 
 class MappedZone(BaseModel):