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>

templates/_course_tree.html

@@ -33,7 +33,7 @@
     </div>
 
     {% if node.children or node.lessons %}
-    <div class="tree-content">
+    <div class="tree-content" data-section-path="{{ node.path }}">
         {% for child_name, child_node in node.children.items() %}
             {{ render_tree_node(child_node, course_path, current_lesson, depth + 1) }}
         {% endfor %}