feat: add feather brush tool with adjustable settings and blending functionality

- Implemented feather brush tool in the brushRaster module, allowing for feathered edges in brush strokes.
- Added new FeatherControls component for UI adjustments of feather settings including size, radius, strength, and smoothing.
- Updated brush preview logic to accommodate feather tool alongside existing brush and eraser tools.
- Enhanced layer rendering to support feather mask previews and interactions.
- Introduced blending logic for feathered strokes to mix blurred mask values with original pixels.
- Added unit tests for feather blending functionality and tool keybindings.
- Updated cursor handling to reflect feather tool usage.

view/toolLabels.ts

@@ -14,6 +14,8 @@ export function labelForTool(tool: ToolId): string {
       return "AI region rectangle";
     case "eraser":
       return "Eraser";
+    case "feather":
+      return "Feather";
     case "pan":
       return "Pan";
     case "select":