feat(app): add AppStateProvider for shared global settings

Central context for the settings a template app shares instead of prop
drilling: theme, viewMode, sidebarCollapsed and the search palette, with
the ⌘K/Ctrl+K shortcut moved out of App.

- settings persist on user action only (an effect-written theme would
  freeze the OS preference for users who never toggled)
- theme still boots from the <html data-theme> attribute the pre-paint
  script set; viewMode mirrors to data-view-mode as the audience-preset
  hook and deliberately gates nothing yet
- Topbar: viewMode segmented control, theme toggle and search now read
  the context; useAppState throws outside the provider
- tests/appState.test.tsx: defaults, persistence, rehydration, garbage
  storage values, ⌘K shortcut

README.md

@@ -46,6 +46,7 @@ src/
   index.css         ← fonts, token import, base resets
   utils.css         ← token-compliant utility classes
   App.css           ← application showcase styles
+  app/              ← AppStateProvider + useAppState(): theme, viewMode, sidebar, ⌘K search
   components/
     ui/             ← composable primitives (Overlay, Dialog, Sheet, Tabs, Alert, …)
     icon-registry.ts← the one icon name → lucide component map