Fix sidebar collapse/expand UX

JS was toggling .collapsed on the wrong element (dashboardSidebar div
instead of lt-sidebar aside), and the expand button was permanently
display:none. When collapsed, users had no way to re-expand.

- toggleSidebar now targets lt-sidebar (the aside)
- Toggle button flips ◀ ↔ ▶ to indicate state and serve as the expand button
- Collapsed CSS hides the body and label, centers the ▶ button in the strip
- Remove the dead sidebarExpandBtn element from HTML
- Persist and restore state correctly on page load

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/base.css

@@ -1450,6 +1450,10 @@ select option:checked {
   transition: var(--transition-default);
 }
 .lt-sidebar.collapsed { width: 32px; overflow: hidden; }
+.lt-sidebar.collapsed .lt-sidebar-body { display: none; }
+.lt-sidebar.collapsed .lt-sidebar-header { justify-content: center; padding: 0.5rem 0; border-bottom: none; }
+.lt-sidebar.collapsed .lt-sidebar-header > span:first-child { display: none; }
+.lt-sidebar.collapsed .lt-sidebar-toggle { font-size: 0.8rem; padding: 0.4rem; color: var(--accent-cyan); }
 
 .lt-sidebar-header {
   display: flex;