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/data/sidebar_prefs.dart

@@ -0,0 +1,23 @@
+// Operator preference: keep the sidebar permanently expanded so
+// every destination shows its text label without hovering. Off by
+// default (hover-expand rail); a11y/senior operators flip it in
+// Settings → Appearance.
+
+import 'package:flutter/foundation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class SidebarPrefs {
+  static const _kPinnedKey = 'sidebar.pinned';
+  static final ValueNotifier<bool> pinned = ValueNotifier(false);
+
+  static Future<void> load() async {
+    final prefs = await SharedPreferences.getInstance();
+    pinned.value = prefs.getBool(_kPinnedKey) ?? false;
+  }
+
+  static Future<void> setPinned(bool value) async {
+    pinned.value = value;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setBool(_kPinnedKey, value);
+  }
+}