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/ToolOverlay.tsx

@@ -1,4 +1,4 @@
-import { Cursor, Eraser, Hand, PaintBrush, DropHalf, MagicWand, Sparkle, Polygon, Rectangle, Selection } from "@phosphor-icons/react";
+import { Cursor, Eraser, Feather, Hand, PaintBrush, DropHalf, MagicWand, Sparkle, Polygon, Rectangle, Selection } from "@phosphor-icons/react";
 import { commandIds } from "@commands/ids";
 import type { AppStore } from "@editor/store";
 import type { InteractionMode, OperationId, ToolId } from "@editor/tools";
@@ -66,6 +66,8 @@ function iconForTool(tool: ToolId) {
       return PaintBrush;
     case "eraser":
       return Eraser;
+    case "feather":
+      return Feather;
     case "magicWand":
       return MagicWand;
     case "semanticSelect":