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/help.html

@@ -173,6 +173,9 @@
                 <li><strong>Space</strong> - play/pause</li>
                 <li><strong>← / →</strong> - seek back/forward 10 seconds</li>
                 <li><strong>↑ / ↓</strong> - volume up/down</li>
+                <li><strong>, / .</strong> - step playback speed down/up</li>
+                <li><strong>[ / ]</strong> - previous/next lesson</li>
+                <li><strong>F</strong> - toggle fullscreen</li>
                 <li><strong>N</strong> - jump into the quick-capture note box (pauses the video)</li>
                 <li>Drag the video's bottom-right corner to resize it, and pick a default playback speed in Settings - both are remembered</li>
                 <li>Reopening a lesson resumes from where you left off, and the <strong>▶ Resume</strong> button at the top of a loaded course jumps straight back to the last lesson you had open</li>