Feature (UI): Add Invert button for Regional Guidance layers

[DustyShoe]

Summary

Added invert support for Regional Guidance by reusing the existing mask inversion flow.
The invert button and hotkey now work for both Inpaint Mask and Regional Guidance, and the button label is context-aware (Invert Mask vs Invert Region).

One minor issue: The feature shares same icon as "Enable Auto Negative"

Related Issues / Discussions

Closes #8875

QA Instructions

1. Select an Inpaint Mask layer, click invert, confirm mask flips and button text is Invert Mask.
2. Select a Regional Guidance layer, click invert, confirm region flips and button text is Invert Region.
3. With each layer type selected, press Shift+V and confirm inversion works the same as button click.
4. Select a non-mask layer (e.g. raster/control), confirm invert button is hidden and hotkey does nothing.

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)