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_en.arb

@@ -1573,6 +1573,9 @@
   "doctorPathStudioErrors": "Studio errors",
   "settingsCategoryGeneral": "General",
   "settingsCategoryAppearance": "Appearance",
+  "sidebarSearchLabel": "Search & commands",
+  "settingsSidebarPinnedTitle": "Keep the navigation expanded",
+  "settingsSidebarPinnedBody": "Shows the sidebar labels permanently instead of only while hovering with the mouse.",
   "settingsCategoryAi": "System AI",
   "settingsCategoryIntegrations": "Integrations",
   "settingsCategorySecurity": "Security",