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.

platform/browser/brushRaster.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, test } from "bun:test";
+import { blendFeatherPatch } from "./brushRaster";
+
+describe("feather brush raster blending", () => {
+  test("mixes blurred mask values inside the stroke and preserves pixels outside it", () => {
+    const original = rgbaRow([255, 255, 255, 255, 255]);
+    const blurred = rgbaRow([0, 64, 128, 192, 255]);
+
+    blendFeatherPatch(original, blurred, 5, 1, { x: 0, y: 0 }, {
+      from: { x: 2.5, y: 0.5 },
+      to: { x: 2.5, y: 0.5 },
+      size: 3,
+      strength: 100,
+    });
+
+    expect(alphaValues(original)).toEqual([255, 64, 128, 192, 255]);
+  });
+
+  test("applies strength as a non-destructive mix", () => {
+    const original = rgbaRow([255]);
+    const blurred = rgbaRow([0]);
+
+    blendFeatherPatch(original, blurred, 1, 1, { x: 0, y: 0 }, {
+      from: { x: 0.5, y: 0.5 },
+      to: { x: 0.5, y: 0.5 },
+      size: 10,
+      strength: 50,
+    });
+
+    expect(alphaValues(original)).toEqual([128]);
+  });
+});
+
+function rgbaRow(alpha: number[]) {
+  return new Uint8ClampedArray(alpha.flatMap((value) => [255, 255, 255, value]));
+}
+
+function alphaValues(data: Uint8ClampedArray) {
+  return Array.from({ length: data.length / 4 }, (_, index) => data[index * 4 + 3]);
+}