Rename the Export stage to Finish

It now holds files, sharing, handouts, slides, importing notes, schedule,
co-teachers and earlier versions, not just exports.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/components/CoTeachers.jsx

@@ -1,4 +1,4 @@
-// Export → Co-teachers: let other accounts edit this study with you. You make a single-use invite link;
+// Finish → Co-teachers: let other accounts edit this study with you. You make a single-use invite link;
 // whoever opens it (signed in) can edit the study, and you can remove them any time. Edits from both of you
 // sync through the account; if you both change the same study, "Combine both" keeps everything.
 import { useCallback, useEffect, useState } from 'react';