feat: enhance layer masking functionality and brush controls

- Refactor LayersSheet component to support mask editing state and improve layer visibility handling.
- Introduce functions to collect mask layer IDs and count display layers excluding masks.
- Update BrushControls to include mask view mode options and a Done button for exiting mask editing.
- Modify brush session handling to support brush previews when editing masks.
- Implement a new BrushPreviewRenderer for rendering brush strokes with visual feedback.
- Add document geometry utilities for transforming points and resolving layer bounds.
- Ensure proper cleanup of brush preview on pointer leave and other interactions.

view/CanvasViewport.tsx

@@ -1,6 +1,7 @@
 import { useMemo, useRef } from "react";
 import type { AppStore } from "@editor/store";
 import type { GlobalKeybindConsumer, GlobalPointerConsumer, GlobalWheelConsumer } from "@input/index";
+import { canPreviewBrush } from "./canvas/brush";
 import { canvasCursorClass } from "./canvas/cursor";
 import { useCanvasInput } from "./canvas/useCanvasInput";
 import { useCanvasRenderer } from "./canvas/useCanvasRenderer";
@@ -34,7 +35,8 @@ export function CanvasViewport({
   useCanvasRenderer(canvasRef, store);
   useCanvasResize(canvasRef, store.dispatch);
   const input = useCanvasInput(canvasRef, store, inputOptions);
-  const cursorClass = canvasCursorClass(state.editor.tools.interactionMode, input);
+  const hasBrushPreview = Boolean(state.editor.brushPreview && canPreviewBrush(state.document, state.editor));
+  const cursorClass = canvasCursorClass(state.editor.tools.interactionMode, input, hasBrushPreview);
 
   return <canvas ref={canvasRef} className={`h-full w-full ${cursorClass}`} />;
 }