Fix(Canvas): Retain layer opacity on mode change.

[DustyShoe]

Summary

This PR fixes canvas layer opacity desync after mode/tab switches. Layer opacity value in state was correct, but visual opacity could remain fully opaque after returning to canvas.

QA Instructions

1. Open Canvas, add a raster layer, set opacity to ~30%.
2. Switch to Generate or Upscale tab, then back to Canvas.
3. Confirm the layer still looks ~30% opacity (not fully opaque).
4. Repeat with multiple raster layers at different opacities.
5. Toggle layer visibility off/on and confirm opacity remains correct.
6. Optional stress check: switch tabs quickly several times and verify no visual opacity reset.

Merge Plan

Simple merge.

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)

[dunkeroni]

Works. Good catch.

To other reviewers: this error only happens for me when I have a raster layer with transparency and different raster layer at full opacity is focused when switching to another tab and back.