feat(ui): inpaint mask extraction to raster layer

[DustyShoe]

Summary

This PR introduces a new feature: Extract Inpaint Mask as a Raster Layer.

The feature allows users to extract the portion of all visible raster layers that falls within the painted Inpaint Mask. The extracted region is composited, masked, and added as a new raster layer.

The new raster layer is currently created at the top of the raster layer stack.
In an ideal workflow, it could appear directly above the active raster layer, but Invoke currently supports only one active raster layer at a time. Changing this behavior may introduce unnecessary complexity; maintainers may consider it separately.

Related Issues / Discussions

QA Instructions

1. Generate or import an image into the canvas so that raster layers are present.

2. Add an Inpaint Mask layer and paint on the mask.

3. In the Layer Panel, right-click the Inpaint Mask layer to open its context menu.

4. Select Extract Masked Area.

5. Verify that:

• A new raster layer is added at the top of the raster layer stack.
• The new layer contains only the composited pixel data within the painted mask region.
• Colors and transparency match the masked area.
• No existing layers are modified.
• No console errors occur.

6. Test with:

• multiple raster layers,
• small and large painted mask regions,
• different zoom/crop states.

Merge Plan

• Wait for CI checks (ESLint, Prettier, TypeScript) to pass.
• Request maintainer review for compositing and extraction logic.
• Optional: discuss potential improvements to the placement of the new raster layer.
• After approval, squash-merge into the target branch.
• No follow-up cleanup required.

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]

Should use translation and get the title from en.json

Also, the current title looks a bit too long in the menu. Maybe something simpler like "Extract Region" or "Create Raster Layer".

[dunkeroni]

From a user standpoint, it's not clear that the operation is restricted to the bounding box or why that would be the case. If I have a mask that exists exclusively outside of the bbox, then this operation returns an empty raster layer without any warnings. Perhaps we can piggyback off of the Merge Visible feature to create a new raster layer of the entire canvas, and then cut that down to match the inpaint mask dimensions and alpha.