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.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, test } from "bun:test";
+import { commandIds } from "@commands/ids";
+import type { Dispatch } from "@commands/dispatcher";
+import { handleToolKey } from "./tool-keybinds";
+
+describe("tool keybinds", () => {
+  test("selects the feather tool with F", () => {
+    const dispatched: unknown[] = [];
+    const consumed = handleToolKey({
+      event: { key: "f", code: "KeyF", altKey: false, ctrlKey: false, metaKey: false, shiftKey: false },
+      dispatch: ((commandId: unknown, payload: unknown) => {
+        dispatched.push({ commandId, payload });
+        return undefined;
+      }) as unknown as Dispatch,
+    });
+
+    expect(consumed).toBe(true);
+    expect(dispatched).toEqual([{ commandId: commandIds.toolSetActive, payload: { tool: "feather" } }]);
+  });
+});