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>
