Fix mobile navigation: visible toggle, overlay, and correct sidebar behavior

frontend/index.html

@@ -12,7 +12,7 @@
         <aside class="sidebar">
             <div class="sidebar-header">
                 <h2>AutoML</h2>
-                <button id="sidebar-toggle"><i class="fas fa-angle-left"></i></button>
+                <button id="sidebar-toggle"><i class="fas fa-bars"></i></button>
             </div>
             <nav class="sidebar-nav">
                 <a href="#" class="nav-link active" data-page="load-dataset"><i class="fas fa-upload"></i><span class="nav-text">Load Dataset</span></a>