Add per-row actions menu (open, duplicate, delete)

[priethor]

Goal

The DataView block has no row-level actions menu yet. You can open a row, but duplicating it means dropping out of the block. Add the basic menu.

Scope

• A per-row menu that shows up on every layout.
• Two actions for now: Open (already there) and Duplicate.
• Duplicate copies the row entirely: title, content, field values, icon, and any relations stay consistent on both sides.
• Delete lands in Add trash and restore for collection rows #179 alongside the trash flow.

Out of scope

• Delete (covered in Add trash and restore for collection rows #179 with trash and restore).
• Bulk selection and bulk delete.
• Bulk edit.
• Manual sort and drag-to-reorder.

Acceptance criteria

• Each row exposes a menu with Open and Duplicate.
• Duplicate creates a new row prefixed "Copy of …", same field values, relations linked both ways.