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>
Export and import encounter, undo/redo history, and player characters
as a downloadable .json file. Export/import actions are in the action
bar overflow menu. Import validates using existing rehydration functions
and shows a confirmation dialog when replacing a non-empty encounter.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
