mobile: off-canvas sidebar, responsive grids/charts/top-bar

- Sidebar becomes slide-in overlay under 768px with backdrop
- Hamburger button in top-bar for mobile nav toggle
- Grids stack single-column, charts shrink to 280px
- Tables scroll horizontally with touch momentum
- Status text hidden on mobile (dot stays visible)
- Card headers/actions stack vertically
- Period selectors scroll horizontally

also: image size validation clamp in handleImageGenerations

static/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>GopherGate - Admin Dashboard</title>
-    <link rel="stylesheet" href="/css/dashboard.css?v=11">
+    <link rel="stylesheet" href="/css/dashboard.css?v=12">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.4.0/css/all.min.css">
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
@@ -51,7 +51,7 @@
                     <span>GopherGate</span>
                 </div>
                 <button class="sidebar-toggle" id="sidebar-toggle">
-                    <i class="fas fa-bars"></i>
+                    <i class="fas fa-times"></i>
                 </button>
             </div>
             
@@ -135,11 +135,17 @@
             </div>
         </nav>
 
+        <!-- Mobile sidebar backdrop -->
+        <div class="sidebar-backdrop" id="sidebar-backdrop"></div>
+
         <!-- Main Content -->
         <main class="main-content">
             <header class="top-bar">
+                <button class="mobile-menu-btn" id="mobile-menu-btn">
+                    <i class="fas fa-bars"></i>
+                </button>
                 <div class="page-title">
-                    <h2 id="current-page-title">Overview</h2>
+                    <h2 id="page-title">Overview</h2>
                 </div>
                 <div class="top-bar-actions">
                     <div id="connection-status" class="status-indicator">