Add free drawing mode for freehand canvas input

[olavl]

Summary

• Adds a draw mode toggle (pencil button) to the preview toolbar that lets users draw freehand on the canvas with mouse or touch
• Mouse-up finalizes the stroke as a new Drawing layer with configurable Douglas-Peucker simplification and Chaikin corner-cutting smoothing
• Drawing layers support free scaling (independent width/height) and all existing transforms and effects

How it works

1. Click the pencil button to enter draw mode (cursor becomes crosshair)
2. Click and drag on the canvas to draw a freehand stroke (live green preview)
3. On mouse-up the stroke becomes a new "Drawing" layer in the sidebar
4. Adjust simplification tolerance and smoothing iterations in the layer options
5. Press Escape to cancel without creating a layer

Files changed

• src/features/shapes/Drawing.js — new shape type with Douglas-Peucker + Chaikin smoothing
• src/features/shapes/shapeFactory.js — register Drawing shape
• src/features/preview/previewSlice.js — drawing mode state and actions
• src/features/preview/previewSlice.spec.js — updated test for new initial state
• src/features/preview/PreviewWindow.js — mouse/touch event handling and live stroke preview
• src/features/preview/PreviewManager.js — draw toggle button and Escape key handling

Test plan

• npm test — all 125 tests pass
• vite build — production build succeeds
• Draw on canvas with mouse, verify layer created
• Draw on canvas with touch, verify layer created
• Verify simplify and smoothing sliders work on Drawing layer
• Verify layer can be moved, scaled (non-uniform), rotated
• Verify effects can be applied to Drawing layers
• Verify GCode export produces valid output
• Verify Escape cancels drawing without creating a layer

🤖 Generated with Claude Code