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/study/StudioShell.jsx

@@ -10,7 +10,7 @@ export const STAGES = [
   { id: 'study', label: 'Study' },
   { id: 'outline', label: 'Outline' },
   { id: 'script', label: 'Script' },
-  { id: 'export', label: 'Export' },
+  { id: 'export', label: 'Finish' },
 ];
 const THEME_ORDER = ['paper', 'sepia', 'dark'];