fix: uncollapse ☰ button on all pages with x-cloak for flicker prevention

Added x-cloak directive and [x-cloak] CSS so button never flashes
before Alpine loads. Button is in base.html so appears on all pages
that extend the base template.

app/templates/base.html

@@ -269,10 +269,14 @@
   </aside>
 
   <!-- Uncollapse button (visible when sidebar hidden) -->
-  <button class="sidebar-uncollapse-btn" x-show="sidebarCollapsed" @click="sidebarCollapsed = false" title="Show sidebar">
+  <button class="sidebar-uncollapse-btn" x-show="sidebarCollapsed" x-cloak @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>
 
+  <style>
+    [x-cloak] { display: none !important; }
+  </style>
+
   <!-- ═══════════ MAIN ═══════════ -->
   <div class="main-wrapper">