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/canvas/renderFrame.test.ts

@@ -78,9 +78,20 @@ describe("canvas render frame selection", () => {
         brushPreview: { position: { x: 10, y: 20 } },
       },
     };
+    const changedFeather = {
+      ...state,
+      editor: {
+        ...state.editor,
+        tools: {
+          ...state.editor.tools,
+          feather: { ...state.editor.tools.feather, size: state.editor.tools.feather.size + 1 },
+        },
+      },
+    };
 
     expect(canvasRenderFramesEqual(selectCanvasRenderFrame(state), selectCanvasRenderFrame(sameSelectionValues))).toBe(true);
     expectFrameChanged(state, changedBrush);
+    expectFrameChanged(state, changedFeather);
     expectFrameChanged(state, changedPreview);
   });