feat(UI): add Inpaint Mask as drag & drop target on canvas

[Pfannkuchensack]

Summary

Add Inpaint Mask as a 5th drag & drop target on the canvas. The grid layout changes from a 2x2 quadrant to a 6-column grid with 2 zones on top and 3 on the bottom:

┌─────────────────────┬─────────────────────┐
│    Raster Layer     │   Control Layer     │
├──────────────┬──────┴───────┬─────────────┤
│ Regional Ref.│ Inpaint Mask │Resized Ctrl │
└──────────────┴──────────────┴─────────────┘

The backend handler (createNewCanvasEntityFromImage) already supported inpaint_mask — this PR just wires up the UI drop zone.

Related Issues / Discussions

Closes #8843

QA Instructions

1. Drag an image from the Gallery onto the Canvas
2. Verify 5 drop zones appear (2 top, 3 bottom)
3. Drop onto "New Inpaint Mask" and confirm a new Inpaint Mask entity is created with the image

Merge Plan

No special considerations — single file change, no migrations needed.

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)

[Pfannkuchensack]