Add an export button to the TabBar header that lets users export
the current session as Markdown, JSON, or Plain Text. The button
appears between Search and Notifications, only for session tabs.
- sessionExporter.ts: formatters for all three formats + download trigger
- ExportDropdown.tsx: dropdown UI component with format selection
- TabBar.tsx: integration with conditional rendering for session tabs
- 51 new tests covering all formatters, edge cases, and download
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
