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>

views/DashboardView.php

@@ -500,12 +500,6 @@ include __DIR__ . '/layout_header.php';
     </div><!-- /.lt-sidebar-body -->
   </aside>
 
-  <!-- Collapsed expand button -->
-  <button type="button" class="lt-sidebar-expand-btn" id="sidebarExpandBtn"
-          data-action="toggle-sidebar"
-          aria-label="Show filters" aria-expanded="false" aria-controls="lt-sidebar"
-          style="display:none">&#x25B6; Filters</button>
-
   <!-- ─── MAIN CONTENT ─────────────────────────────────────── -->
   <div class="lt-content">