feat(ui): add collapsible sidebar for mobile devices

- Add hamburger menu toggle button in top bar (visible on mobile only)
- Sidebar slides in/out from the left with smooth transition
- Add dark overlay when sidebar is open on mobile
- Close sidebar on overlay click, nav item click, or escape key
- Support RTL languages (sidebar slides from right)
- Add dark theme styles for toggle button
- Responsive breakpoint at 768px

static/css/themes/dark.css

@@ -201,3 +201,13 @@
 [data-theme="dark"] .search-sort-select:focus {
     border-color: #ff5e3a;
 }
+
+/* Dark theme for mobile sidebar toggle */
+[data-theme="dark"] .sidebar-toggle {
+    color: #94a3b8;
+}
+
+[data-theme="dark"] .sidebar-toggle:hover {
+    background-color: #334155;
+    color: #ff5e3a;
+}