feat: warn before losing timetable changes

[condyl]

Summary

• Warn before leaving the page while a populated timetable has not been copied as a link.
• After a successful copy, compare the copied s snapshot with the current URL state so later timetable changes restore the warning.
• Keep browser-native prompt wording and do not treat calendar export as copying a share link.

Verification

• Pre-commit formatting passed.
• ESLint passed.
• Vitest passed: 12 files, 33 tests.