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/pages/welcome.dart

@@ -444,10 +444,13 @@ class _PillarState extends State<_Pillar> {
                     color: theme.colorScheme.primary,
                   ),
                   const SizedBox(width: 4),
-                  Text(
-                    l.welcomePillarTechToggle,
-                    style: theme.textTheme.labelMedium?.copyWith(
-                      color: theme.colorScheme.primary,
+                  Flexible(
+                    child: Text(
+                      l.welcomePillarTechToggle,
+                      overflow: TextOverflow.ellipsis,
+                      style: theme.textTheme.labelMedium?.copyWith(
+                        color: theme.colorScheme.primary,
+                      ),
                     ),
                   ),
                 ],