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>

test/sidebar_test.dart

@@ -41,6 +41,12 @@ void main() {
   testWidgets(
     'sidebar destination Y positions are stable across rail expansion',
     (tester) async {
+      // Desktop-sized surface: the rail carries 8 destinations plus
+      // the search row and footer — the 600-px default viewport
+      // scrolls the last destination out of the ListView.
+      tester.view.physicalSize = const Size(1280, 900);
+      tester.view.devicePixelRatio = 1.0;
+      addTearDown(tester.view.reset);
       // Collapsed (default).
       await tester.pumpWidget(
         const StudioApp(