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/l10n/app_localizations_en.dart

@@ -2668,6 +2668,16 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get settingsCategoryAppearance => 'Appearance';
 
+  @override
+  String get sidebarSearchLabel => 'Search & commands';
+
+  @override
+  String get settingsSidebarPinnedTitle => 'Keep the navigation expanded';
+
+  @override
+  String get settingsSidebarPinnedBody =>
+      'Shows the sidebar labels permanently instead of only while hovering with the mouse.';
+
   @override
   String get settingsCategoryAi => 'System AI';