[UI parity] Implement Insert Space as a draggable canvas spacing tool rather than inserting one blank paragraph #164

Tested behavior

Main 33429876e885613f6ff8f29a762b15b2be736401, Windows, real UI harness. Open a paragraph followed by a table, focus the paragraph and click Insert > Insert Space.

Actual: a single empty paragraph is inserted immediately after the caret. There is no spacing cursor, drag gesture, preview, or way to close a gap. Separate containers and ink are not moved as a group by the tool. app.rs::Act::Insert("space") directly inserts Block::Paragraph(Vec::new()).

OneNote comparison

Microsoft's Insert extra writing space describes choosing the tool and dragging on the page to add or remove space between content.

Requested behavior

Turn Insert Space into a canvas tool: show a directional cursor/guide at the proposed boundary, let the user drag a distance, move affected containers/objects and ink together, and support dragging upward to remove a gap. Keep unaffected content stable, offer Cancel/Esc, and make the operation undoable.

This is a major interaction difference despite using the same ribbon name/icon. Retain an ordinary blank-paragraph insertion action if useful, with an appropriate label.

Checked existing issues/PRs: #23 is an overall drawing collection request, #72 concerns container merging. The separately reported missing Undo for the current command does not provide this canvas spacing workflow.