Improve mobile responsiveness with hamburger menu and drawer sidebar

app/globals.css

@@ -1085,6 +1085,46 @@ tr:hover td {
   background: var(--danger-dim);
 }
 
+/* ── Mobile Header ── */
+.mobile-header {
+  display: none;
+  align-items: center;
+  gap: 16px;
+  padding: 12px 16px;
+  background: var(--bg-card);
+  border-bottom: 1px solid var(--border);
+  position: sticky;
+  top: 0;
+  z-index: 50;
+}
+
+.mobile-menu-btn {
+  background: none;
+  border: none;
+  color: var(--text-primary);
+  cursor: pointer;
+  padding: 4px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+.mobile-logo {
+  font-weight: 700;
+  font-size: 16px;
+  color: var(--accent-hover);
+}
+
+.sidebar-overlay {
+  display: none;
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.5);
+  backdrop-filter: blur(4px);
+  z-index: 90;
+  animation: fadeIn 0.2s ease;
+}
+
 /* ── Responsive ── */
 @media (max-width: 1024px) {
   .mail-layout { grid-template-columns: 60px 280px 1fr; }
@@ -1097,12 +1137,42 @@ tr:hover td {
 }
 
 @media (max-width: 768px) {
-  .sidebar { display: none; }
-  .mail-layout { grid-template-columns: 1fr; }
+  .mobile-header { display: flex; }
+  
+  .sidebar { 
+    position: fixed;
+    left: -240px;
+    top: 0;
+    bottom: 0;
+    z-index: 100;
+    transition: transform 0.3s cubic-bezier(0.4, 0, 0.2, 1);
+    box-shadow: 10px 0 30px rgba(0,0,0,0.5);
+  }
+  
+  .sidebar-open .sidebar {
+    transform: translateX(240px);
+  }
+  
+  .sidebar-open .sidebar-overlay {
+    display: block;
+  }
+
+  .mail-layout { grid-template-columns: 1fr; height: auto; overflow: visible; }
   .mail-sidebar { display: none; }
   .mail-detail { display: none; }
+  
+  /* Show active mail view if selected */
+  .mail-view-active .mail-list { display: none; }
+  .mail-view-active .mail-detail { display: block; }
+
   .page-body { padding: 16px; }
-  .page-header { padding: 16px; }
+  .page-header { 
+    padding: 16px; 
+    flex-direction: column; 
+    align-items: flex-start;
+  }
+  .page-header .btn { width: 100%; justify-content: center; }
+  
   .stats-grid { grid-template-columns: 1fr 1fr; }
 }
 /* ── Language Switcher ── */