Move File Management link into the bottom tab bar

Replace the dashboard-only footer link with a "Files" item in the
persistent bottom tab bar (Home/Notes/Help/Settings) on every page, so
it's reachable the same way everywhere instead of only from the
dashboard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

offlineu_core.py

@@ -3588,7 +3588,7 @@ def notes_hub():
 @app.route('/unsorted')
 def unsorted_page():
     """Review page for filing new courses out of the Unsorted folder - see /api/unsorted/scan for the actual proposals."""
-    return render_template('unsorted.html', active_tab=None)
+    return render_template('unsorted.html', active_tab='files')
 
 
 @app.route('/library/search-transcripts')