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/ImportNotes.jsx

@@ -1,4 +1,4 @@
-// Export → Import notes: bring notes from a Word or Markdown file into this study, matched to its chunks by passage.
+// Finish → Import notes: bring notes from a Word or Markdown file into this study, matched to its chunks by passage.
 import { useState } from 'react';
 import { useApp } from '../context/AppContext.js';
 import { useToast } from './Toasts.jsx';