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_dashboard.html

@@ -1360,29 +1360,49 @@
         }
 
         function toggleTree(element) {
-            console.log('Toggle clicked:', element);
-            
             // Find the content div that comes after this header
             const content = element.nextElementSibling;
             const toggle = element.querySelector('.tree-toggle');
-            
-            console.log('Content element:', content);
-            console.log('Toggle element:', toggle);
-            
+
             if (content && content.classList.contains('tree-content')) {
-                const isExpanded = content.classList.contains('expanded');
-                content.classList.toggle('expanded');
-                
-                if (toggle) {
-                    toggle.textContent = content.classList.contains('expanded') ? '▼' : '▶';
-                }
-                
-                console.log('Toggled tree, expanded:', content.classList.contains('expanded'));
-            } else {
-                console.log('No content element found or not tree-content');
+                const isExpanded = content.classList.toggle('expanded');
+                if (toggle) toggle.textContent = isExpanded ? '▼' : '▶';
+                if (content.dataset.sectionPath) saveSectionExpanded(content.dataset.sectionPath, isExpanded);
             }
         }
 
+        // Remembers which course sections a user has manually expanded
+        // (keyed by the section's absolute path, so it's stable across
+        // page loads and shared between this tree and the lesson page's
+        // sidebar - see _course_tree.html) so re-opening a course doesn't
+        // reset every section back to collapsed.
+        function getExpandedSectionPaths() {
+            try {
+                return new Set(JSON.parse(localStorage.getItem('offlineu-expanded-sections') || '[]'));
+            } catch (e) {
+                return new Set();
+            }
+        }
+
+        function saveSectionExpanded(path, expanded) {
+            const paths = getExpandedSectionPaths();
+            if (expanded) paths.add(path); else paths.delete(path);
+            localStorage.setItem('offlineu-expanded-sections', JSON.stringify([...paths]));
+        }
+
+        function restoreExpandedSections() {
+            const saved = getExpandedSectionPaths();
+            if (!saved.size) return;
+            document.querySelectorAll('.tree-content[data-section-path]').forEach(function(content) {
+                if (!saved.has(content.dataset.sectionPath)) return;
+                content.classList.add('expanded');
+                const header = content.previousElementSibling;
+                const toggle = header ? header.querySelector('.tree-toggle') : null;
+                if (toggle) toggle.textContent = '▼';
+            });
+        }
+        restoreExpandedSections();
+
         // Single-level drill-down: the Library browser shows one folder's
         // contents at a time (replacing the list, not nesting under it) and
         // tracks how we got here in libraryTrail so a breadcrumb can jump