Move sidebar left, add collapse toggle and remembered sections

- Course outline sidebar now sits on the left of the player instead
  of the right (mobile stacking order unchanged - video still first).
- Collapsible to a slim rail via a toggle button, remembered per
  device, for a full-width player when wanted.
- Which sections a user has manually expanded is now remembered
  (keyed by the section's path) and shared between the lesson-page
  sidebar and the dashboard's course view, so re-opening a course
  doesn't reset everything back to collapsed.

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

README.md

@@ -225,12 +225,15 @@ 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
+- Course outline sidebar on the lesson page (left side): 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.
+  Collapsible to a slim rail (remembered per device) for a full-width
+  player, and which sections you've manually expanded is remembered too,
+  so re-opening a course doesn't reset everything back to collapsed.
   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