feat(gallery): add masonry gallery layout mode

[AsuraAce]

Summary

Adds a new gallery layout mode for browsing images in a masonry layout while preserving the existing square grid as the default.

This PR adds:

• A persisted grid | masonry gallery layout setting, defaulting to grid with a persisted-state migration.
• A masonry-only non-paged gallery path with DTO batching, thumbnail preloading, keyboard navigation scroll handling, and static masonry rendering for smaller boards.
• Header icon controls for switching between grid and masonry layouts.
• Explicit behavior that paged gallery remains grid-only and selecting masonry disables paged gallery.

View screenshots

Masonry View:

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Grid (Regular Default)

Related Issues / Discussions

N/A

QA Instructions

Validated locally with:

• pnpm --dir invokeai/frontend/web exec vitest run src/features/gallery/components/GalleryImageGridMasonry.test.ts src/features/gallery/components/galleryLayout.test.ts src/features/gallery/store/types.test.ts
• pnpm --dir invokeai/frontend/web exec tsc --noEmit
• pnpm --dir invokeai/frontend/web run lint:eslint
• pnpm --dir invokeai/frontend/web run lint:prettier
• pnpm --dir invokeai/frontend/web run lint:dpdm

Manual browser QA covered:

• Switching between grid and masonry from the gallery header.
• Masonry scrolling with large boards and mixed aspect ratios.
• Keyboard navigation and alt-compare navigation in masonry.
• Paged gallery remaining grid-only.
• Header remaining visible when switching layout modes after scrolling.

Merge Plan

No special merge handling expected.

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)