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/App.tsx

@@ -220,11 +220,12 @@ export function App({ app }: AppProps) {
           document={document}
           selection={selection}
           viewport={viewport}
-          visible={generateOpen || chromaKeyOpen || tools.activeTool === "brush" || tools.activeTool === "eraser" || tools.activeTool === "magicWand" || tools.activeTool === "semanticSelect" || tools.activeTool === "maskLasso" || tools.activeTool === "maskRectangle" || Boolean(transformBounds) || viewportActivityIsland.visible}
+          visible={generateOpen || chromaKeyOpen || tools.activeTool === "brush" || tools.activeTool === "eraser" || tools.activeTool === "feather" || tools.activeTool === "magicWand" || tools.activeTool === "semanticSelect" || tools.activeTool === "maskLasso" || tools.activeTool === "maskRectangle" || Boolean(transformBounds) || viewportActivityIsland.visible}
           action={viewportActivityIsland.action}
           activeTool={tools.activeTool}
           operation={generateOpen ? "generate" : chromaKeyOpen ? "chromaKey" : undefined}
           brushSettings={tools.brush}
+          featherSettings={tools.feather}
           generateSettings={tools.generate}
           generation={generation}
           chromaKeySettings={tools.chromaKey}