The context menu advertised Ctrl+C/Ctrl+V hints but useKeyboardShortcuts
only handled undo/redo/delete/duplicate/escape -- pressing them did
nothing. Add a tiny shared module-level clipboard (src/hooks/clipboard.ts)
used by both the keyboard hook and the context menu so copying via one
entry point and pasting via the other stay consistent.
Ctrl/Cmd+C stores the selected node id (skipping ROOT). Ctrl/Cmd+V
inserts a copy as a sibling of the current selection via
regenerateTreeIds, mirroring the existing context-menu paste behavior.
Both respect the existing "disabled while typing" guard.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
