Add export dialog with download/clipboard options and optional
undo/redo history inclusion (default off). Extract shared Dialog
component to ui/dialog.tsx, consolidating open/close lifecycle,
backdrop click, and escape key handling from all 6 dialog components.
Update spec to reflect export method dialog and optional history.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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