feat(studio): add conversation export formats

[evanlowe]

Summary

• add PNG and PDF options to the Studio conversation export dialog
• paginate PDF exports on A4 pages while avoiding content-line clipping
• show the dialog and loading state before generating long conversation previews
• remove the redundant cancel action while keeping close and Escape support

Validation

• uv run pre-commit run --all-files
• npm test (939 tests)
• npm run build
• visual QA at 1200x889 and 390x844 for loading and ready states
• verified long-conversation dialog/loading appears before capture work starts