Add expand all/collapse all to the course tree

An "Expand all" link above the tree (toggling to "Collapse all" once
everything's open) opens or closes every section at once - on both
the lesson page's sidebar and the dashboard's loaded-course view.
Toggles stay right-aligned per section, matching convention rather
than moving to the left. Label reflects current state on load and
persists through the same remembered-sections mechanism as manual
expand/collapse.

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

README.md

@@ -238,7 +238,9 @@ silently stay blank instead of erroring.
   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.
+  so re-opening a course doesn't reset everything back to collapsed. An
+  "Expand all" link (toggling to "Collapse all") opens or closes every
+  section at once - same control on the dashboard's course tree.
   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