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/lesson_view.html

@@ -1291,6 +1291,18 @@
                 return tag === 'TEXTAREA' || tag === 'INPUT' || tag === 'SELECT' || el.isContentEditable;
             }
 
+            const PLAYBACK_SPEEDS = ['0.75', '1', '1.25', '1.5', '1.75', '2'];
+
+            function stepPlaybackSpeed(direction) {
+                if (!activeMedia) return;
+                let idx = PLAYBACK_SPEEDS.findIndex(s => parseFloat(s) === activeMedia.playbackRate);
+                if (idx === -1) idx = PLAYBACK_SPEEDS.indexOf('1');
+                idx = Math.max(0, Math.min(PLAYBACK_SPEEDS.length - 1, idx + direction));
+                const speed = PLAYBACK_SPEEDS[idx];
+                setPlaybackSpeed(speed);
+                showNotification(`Speed: ${speed}x`);
+            }
+
             document.addEventListener('keydown', function(e) {
                 if (isTypingTarget(e.target)) return;
                 if ((e.key === 'n' || e.key === 'N') && !e.ctrlKey && !e.altKey && !e.metaKey) {
@@ -1298,6 +1310,33 @@
                     openQuickCapture();
                     return;
                 }
+                if (!e.ctrlKey && !e.altKey && !e.metaKey) {
+                    switch(e.key) {
+                        case '[':
+                            {% if prev_lesson %}
+                            e.preventDefault();
+                            window.location.href = '/lesson/{{ prev_lesson }}';
+                            {% endif %}
+                            break;
+                        case ']':
+                            {% if next_lesson %}
+                            e.preventDefault();
+                            window.location.href = '/lesson/{{ next_lesson }}';
+                            {% endif %}
+                            break;
+                        case 'f':
+                        case 'F':
+                            if (video) {
+                                e.preventDefault();
+                                if (document.fullscreenElement) {
+                                    document.exitFullscreen();
+                                } else {
+                                    video.requestFullscreen();
+                                }
+                            }
+                            break;
+                    }
+                }
                 if (activeMedia && !e.ctrlKey && !e.altKey && !e.metaKey) {
                     switch(e.key) {
                         case ' ':
@@ -1326,6 +1365,14 @@
                             activeMedia.volume = Math.max(0, activeMedia.volume - 0.1);
                             showNotification(`Volume: ${Math.round(activeMedia.volume * 100)}%`);
                             break;
+                        case ',':
+                            e.preventDefault();
+                            stepPlaybackSpeed(-1);
+                            break;
+                        case '.':
+                            e.preventDefault();
+                            stepPlaybackSpeed(1);
+                            break;
                     }
                 }
             });