feat(ui): add new paste below action on arrays

[paulpopus]

Summary

Array and Blocks field rows had a "Paste Row" action that overwrote the entire target row. The label and icon did not show this. There was no way to insert a copied row below a target row without losing its data.

How

• Renamed the per-row paste action to "Replace Row" and switched its icon to a swap icon. This makes the overwrite behavior clear.
• Added a "Paste Below" action with a down arrow icon. It inserts the copied row as a new row directly below the target row, without changing the target row or any other row.
• "Paste Below" reuses the existing clipboard merge logic. It first makes room for a new row, then merges the clipboard data into it, so nested ID regeneration works the same way it does for "Replace Row".
• "Paste Below" is hidden when the field is at its max row count, matching the existing behavior of "Add Below" and "Duplicate".
• The field-level "Paste Field" action in the field header is unchanged. It still pastes below all items.

[github-actions]

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