feat(nav): discoverable sidebar — instant tooltips, shortcuts made visible, pinnable rail

The icon-only rail forced first-time users to guess (usertest:
Senior, a11y, UX personas). Three changes:
- Nav tooltips appear instantly and carry the page shortcut
  (Cmd+1..9, Ctrl on non-mac — Ctrl activators added); expanded
  labels show the same hint. Explicit button semantics for
  screen readers on every destination.
- A visible 'Search & commands' row above the footer opens the
  existing Cmd+K palette, which nothing in the UI advertised.
- Settings -> Appearance gains 'Keep the navigation expanded':
  pins the rail with permanent labels (persisted preference).
Footer strip and pillar toggle made overflow-safe for the
animating rail; responsive test scrolls the by-design scrollable
destinations list.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/widgets/chain_settings_dialog.dart

@@ -9,6 +9,7 @@ import '../data/error_presentation.dart';
 import '../data/hub.dart';
 import '../data/hub_auth_token.dart';
 import '../data/registry_token.dart';
+import '../data/sidebar_prefs.dart';
 import '../data/system_actions.dart';
 import '../l10n/app_localizations.dart';
 import '../pages/welcome.dart' show showFaiDoc;
@@ -604,6 +605,19 @@ class _FaiSettingsDialogState extends State<ChainSettingsDialog> {
         theme,
       ),
       const _ThemePluginPanel(),
+      const SizedBox(height: ChainSpace.xl),
+      // Pinned rail: permanent sidebar labels for operators who
+      // don't want (or can't use) the hover-expand gesture.
+      ValueListenableBuilder<bool>(
+        valueListenable: SidebarPrefs.pinned,
+        builder: (context, pinned, _) => SwitchListTile(
+          contentPadding: EdgeInsets.zero,
+          title: Text(l.settingsSidebarPinnedTitle),
+          subtitle: Text(l.settingsSidebarPinnedBody),
+          value: pinned,
+          onChanged: (v) => SidebarPrefs.setPinned(v),
+        ),
+      ),
     ];
   }