feat(canvas): add per-layer lock transparency option

[Pfannkuchensack]

Summary

Add a "Lock Transparency" toggle to raster layers that preserves the alpha channel when painting. When enabled, brush strokes use source-atop compositing to only paint on existing non-transparent pixels, similar to Photoshop's "Lock Transparent Pixels" feature.

Closes #7321

Related Issues / Discussions

• #7321 - Lock transparency on layers

QA Instructions

1. Create a raster layer and paint something on it
2. Toggle the new "Lock Transparency" button (drop icon) in the raster layer header
3. Switch to a different color and paint over the layer:
   • Brush should only paint where existing non-transparent pixels are
   • Painting on transparent areas should have no effect
4. Toggle lock transparency off — normal painting should resume
5. Test with both regular brush and pressure-sensitive pen input
6. Test the onStagePointerEnter path: start painting, drag cursor off canvas, drag back on — stroke should respect transparency lock
7. Verify undo/redo works correctly with locked transparency strokes

Merge Plan

No special merge considerations. isTransparencyLocked is an optional field, so existing state without it will default to undefined/false.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)