Description

Summary
Add a keyboard shortcut for duplicating blocks in the document editor and show that shortcut in the block action menu.

Current behavior
The document block action menu has a Duplicate action, but it can only be triggered from the menu. There is no keyboard shortcut for it.

Expected behavior
Cmd + D duplicates the current block on macOS
Ctrl + D duplicates the current block on Windows/Linux
If multiple blocks are selected, the shortcut duplicates all selected blocks
The block action menu shows the shortcut next to Duplicate
The shortcut appears in Settings > Shortcuts

Impact

This makes duplication faster for desktop users and keeps the editor more consistent with other shortcut-driven actions.

Additional Context

No response