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.dart

@@ -4544,6 +4544,24 @@ abstract class AppLocalizations {
   /// **'Appearance'**
   String get settingsCategoryAppearance;
 
+  /// No description provided for @sidebarSearchLabel.
+  ///
+  /// In en, this message translates to:
+  /// **'Search & commands'**
+  String get sidebarSearchLabel;
+
+  /// No description provided for @settingsSidebarPinnedTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Keep the navigation expanded'**
+  String get settingsSidebarPinnedTitle;
+
+  /// No description provided for @settingsSidebarPinnedBody.
+  ///
+  /// In en, this message translates to:
+  /// **'Shows the sidebar labels permanently instead of only while hovering with the mouse.'**
+  String get settingsSidebarPinnedBody;
+
   /// No description provided for @settingsCategoryAi.
   ///
   /// In en, this message translates to: