fix: uncollapse button ☰ appears when sidebar is hidden

When sidebarCollapsed=true, a hamburger button appears at top-left
corner to restore the sidebar. CSS fixed positioning outside sidebar
so it's always visible even when sidebar is collapsed.

app/templates/base.html

@@ -268,6 +268,11 @@
     </div>
   </aside>
 
+  <!-- Uncollapse button (visible when sidebar hidden) -->
+  <button class="sidebar-uncollapse-btn" x-show="sidebarCollapsed" @click="sidebarCollapsed = false" title="Show sidebar">
+    <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="3" y1="6" x2="21" y2="6"/><line x1="3" y1="12" x2="21" y2="12"/><line x1="3" y1="18" x2="21" y2="18"/></svg>
+  </button>
+
   <!-- ═══════════ MAIN ═══════════ -->
   <div class="main-wrapper">