fix(collection): add spacing above the add item button in column layouts

[GermainBergeron]

Proposed Changes

Adds 16px (--mantine-spacing-sm) between the last row and the Add item button of Collection column layouts, which previously had no spacing. The legacy (children) layout is unchanged since its gap already spaces the button.

Potential Breaking Changes

None.

Acceptance Criteria

• The proposed changes are covered by unit tests
• The potential breaking changes are clearly identified
• A changeset is added for releasable changes, following CONTRIBUTING.md (covered by gentle-comets-wander.md)
• README.md is adjusted to reflect the proposed changes (if relevant)

[svcsnykcoveo]

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[github-actions]

Release Preview

Warning: Changesets detected package changes since origin/master, but no changeset file was found.

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[github-actions]

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[Copilot]

🟡 Changes recommended

The new spacing behavior lacks the required test coverage.

1 open finding

• Add tests for spacing in column and legacy children layouts · New

What changed in this PR

Adds spacing between column-layout rows and the Collection add-item button while preserving legacy layout spacing.

Changes:

• Adds sm top margin to direct-child add-button containers.
• Limits the rule to column layouts.

File	Description
Collection.module.css	Adds column-layout add-button spacing.

🧠 Review effort: Balanced

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