Add course outline sidebar to the lesson page

Shows every section and lesson in the course - videos and standalone
documents alike - so jumping to a different section no longer means
backing out to the course page first. Current lesson is highlighted,
its section auto-expands and scrolls into view; sticky on desktop,
stacks below the player on narrow viewports. Replaces the old
"Lessons in this section" list, which only showed the current
section.

The tree-rendering markup (shared with the loaded-course dashboard
view) is now a single macro in templates/_course_tree.html instead of
being duplicated, so both views stay in sync going forward.

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

README.md

@@ -225,6 +225,14 @@ silently stay blank instead of erroring.
 **Playback & progress**
 - Video/audio player with resize, playback-speed presets, and resume-from-
   last-position
+- Course outline sidebar on the lesson page: every section and lesson in
+  the course (videos and documents alike - a standalone PDF/doc is its own
+  entry, same as a video), one click to jump anywhere without backing out
+  to the course page first. The current lesson's section opens
+  automatically and scrolls into view; sticky and independently
+  scrollable on desktop, stacks below the player on narrow viewports.
+  Shares its rendering with the loaded-course dashboard view (`templates/
+  _course_tree.html`), so the two always look and behave the same.
 - Auto-play next lesson when one ends, with a cancelable few-second
   countdown - on by default, toggle it off in Settings → Video Player
 - Keyboard shortcuts on the lesson page: Space (play/pause), ←/→ (seek