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.

input/tool-keybinds.ts

@@ -5,6 +5,7 @@ import type { KeybindEvent } from "./keyboard";
 const toolKeybinds = {
   b: "brush",
   e: "eraser",
+  f: "feather",
   w: "magicWand",
   p: "pan",
   s: "select",