Add sidebar toggle button to breadcrumb with open/close functionality

Move sidebar toggle from sidebar panel to breadcrumb navigation, positioned
left of the HOME icon. Uses distinct icons for open (sidebar-inset) and
closed (sidebar) states. Sidebar state persists via sessionStorage. Remove
old non-functional toggle buttons from layout and HTMLBlock plugin.

engine/templates/layout.mustache

@@ -127,6 +127,37 @@
             text-decoration: underline;
         }
         
+        /* Sidebar toggle button in breadcrumb */
+        .sidebar-toggle-item {
+            display: flex;
+            align-items: center;
+            margin-right: 0.5rem;
+        }
+        
+        .sidebar-toggle-btn {
+            padding: 0;
+            line-height: 1;
+            font-size: 1.1rem;
+            color: var(--header-bg) !important;
+            border: none !important;
+            background: transparent !important;
+            box-shadow: none !important;
+            cursor: pointer;
+        }
+        
+        .sidebar-toggle-btn:hover {
+            opacity: 0.7;
+        }
+        
+        /* Sidebar hide/show transition */
+        .sidebar-column {
+            transition: all 0.3s ease;
+        }
+        
+        .sidebar-hidden {
+            display: none !important;
+        }
+        
         /* Navigation section background */
         .navigation-section {
             background-color: var(--nav-bg) !important;