Add Favorites, storage drill-down, keyboard shortcuts, and What's New

- Favorites: star toggle on course rows, the course page, and a new
  dashboard card; rebased on rename/move, backed up/restored, and
  caught by stale-reference cleanup (also fixed a pre-existing gap
  where favorites.json wasn't in the backup file list).
- Storage Usage: click a folder row to drill into its contents one
  level at a time, with a Back button.
- Lesson player: added ,/. (speed step), [/] (prev/next lesson), and
  F (fullscreen) keyboard shortcuts.
- Settings: a CHANGELOG.md-backed "What's New" list, since VERSION
  alone only ever shows the current build.

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

templates/settings.html

@@ -470,6 +470,17 @@
         <p style="text-align: center; color: var(--text-muted); font-size: 0.8em; margin-top: 30px;">
             Version {{ build_version }}
         </p>
+
+        {% if changelog_entries %}
+        <details style="max-width: 500px; margin: 10px auto 0; color: var(--text-muted); font-size: 0.8em;">
+            <summary style="text-align: center; cursor: pointer;">What's New</summary>
+            <ul style="margin: 8px 0 0; padding-left: 20px;">
+                {% for entry in changelog_entries %}
+                <li style="margin-bottom: 4px;">{{ entry }}</li>
+                {% endfor %}
+            </ul>
+        </details>
+        {% endif %}
     </div>
     </div>