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/HandoutExport.jsx

@@ -1,4 +1,4 @@
-// Export → Lesson handout: a printable page for each session (or one session), for a group to follow along.
+// Finish → Lesson handout: a printable page for each session (or one session), for a group to follow along.
 import { useState } from 'react';
 import { HANDOUT_LAYOUTS, printHandout, sessionsFor } from '../utils/handout.js';