Add row removal and lane toggle to the edit session

[tmustier]

What

Two new fixed shortcuts while a queue row is selected, covering the two missing row intents: cancelling a queued message and changing steering vs follow-up.

• Option+X marks the selected row for removal. Save deletes it — finally including image-only rows — and Escape or a second press restores it.
• Option+T toggles the selected row between steering and follow-up as a session draft. The row previews at its destination lane's tail (with a moves here on save note) before the save commits it.

Design notes

• Both verbs are drafts inside the existing snapshot edit session: Escape rolls back everything, and touched heads stay pinned, so a row being cancelled can never dispatch out from under the cursor.
• Saves still never change a row's lane implicitly — Enter/Option+Enter keep their save-in-place meaning; only the explicit toggle re-lanes.
• Committed re-lanes join the destination lane's tail, keeping FIFO honest; commit order matches the preview order.
• Row navigation now follows the visual timeline, so Option+Up/Option+Down stay aligned with what's on screen while a lane draft previews a row in the other box.
• Restore-to-composer was considered and rejected: it round-trips one-keystroke intents, drops images (setEditorText cannot reattach them), and muddies Escape/rollback semantics.

Verification

• npm run ci green (39 tests, 10 new: removal marks, lane toggles, rollback, head pinning, visual navigation, commit ordering).
• Visually verified in a real Pi session (tmux-driven, Pi 0.80.9): edit-mode help line, lane preview + undo, removal mark + undo, full Escape rollback, committed re-lane to steering tail, committed removal with notification.