feat(mosaic): support multiple mosaic styles (pixel, blur, frosted, solid)

[xysamuel]

Summary

This PR adds support for 5 distinct mosaic styles:

• Box · Pixelate (框选 · 像素马赛克): Drag a rectangular region with crisp pixelation blocks.
• Box · Blur (框选 · 高斯模糊): Drag a rectangular region with smooth Gaussian blur.
• Box · Frosted Glass (框选 · 毛玻璃): Drag a rectangular region with a milky veil and hairline border.
• Box · Solid Color (框选 · 色块遮挡): Drag a solid color block matching the chosen palette color.
• Brush · Pixelate (涂抹 · 像素马赛克): Classic freehand pixelate brush stroke.

Highlights

1. Style Selection:
   • Added style popup selector in the capture overlay toolbar when mosaic is active.
   • Added style dropdown in the background image editor toolbar.
   • Preserves user choice across sessions via editor.mosaicStyle.
2. Restyling:
   • Selecting an existing mosaic annotation and changing the style dropdown seamlessly restyles it while preserving coverage bounds.
3. History Compatibility:
   • Adds mosaicStyle to Annotation coding keys.
   • Backwards compatible: older entries without mosaicStyle automatically fall back to .brush. Unknown future styles safely fallback without failing decoding.
4. Testing:
   • Includes full unit test suite MosaicStyleTests.swift covering rendering distinction, solid coloring, blur contrast, history serialization/fallback, restyle conversions, and editor undo/drag behavior.
   • All tests pass via ./scripts/test-mosaic.sh.