feat: Add Mosaic (pixelation) tool to annotation system

[Bae-ChangHyun]

Summary

• Add a new Mosaic tool (M) to the annotation system that allows users to pixelate/censor regions of an image by dragging a rectangle
• Useful for hiding sensitive information (faces, text, personal data) before sharing screenshots
• Fully integrated with existing undo/redo, export, presets, delete, and history systems

How it works

1. Press M or click the Mosaic button (grid icon) to enter mosaic mode
2. Click and drag a rectangle over the area to pixelate
3. Adjust Block Size (5, 10, 15, 20, 30px) to control pixelation intensity
4. Mosaic regions behave like annotations: selectable, movable, resizable, deletable
5. Save flattens mosaic permanently into the image

Technical details

• New file: src/ImageAnnotation/tools/MosaicTool.ts — Core mosaic logic with __isMosaicRegion marker to distinguish mosaic FabricImage objects from the background image
• Pixelation algorithm: Extract region → downscale → nearest-neighbor upscale (imageSmoothingEnabled = false)
• Edge cases handled: Background bounds clamping, tiny drag ignore (<3px), spacebar panning override, viewport zoom via getScenePoint()

Files changed

File	Change
src/ImageAnnotation/tools/MosaicTool.ts	NEW — MosaicTool class, MOSAIC_MARKER, isMosaicImage()
src/ImageAnnotation/types.ts	Add ToolMode.MOSAIC, MOSAIC_BLOCK_SIZES
src/ImageAnnotation/managers/ToolManager.ts	Mosaic mode state, toggle, interactivity
src/ImageAnnotation/ui/ToolbarBuilder.ts	Mosaic button + block size controls
src/ImageAnnotation/ImageAnnotationModal.ts	MosaicTool integration, M hotkey, mouse events
src/ImageAnnotation/managers/HistoryManager.ts	Mosaic marker serialization/restore
src/ImageAnnotation/export/ImageExporter.ts	Index-based background detection (avoid mosaic confusion)
src/ImageConverterSettings.ts	Mosaic presets
styles.css	Block size controls styles
docs/Annotation tool.md	Mosaic section + screenshot
tests/	Updated mocks for mosaic compatibility

Test plan

• npm run build — passes
• npm run lint — 0 errors
• npm run test — all annotation tests pass (pre-existing unrelated failures only)
• Manual test: M key → drag rectangle → pixelation applied
• Manual test: Block size change → re-drag → different pixelation
• Manual test: Ctrl+Z/Y undo/redo works with mosaic regions
• Manual test: Select mosaic → Delete removes it
• Manual test: Save → image file has mosaic baked in