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

AGENTS.md

@@ -35,6 +35,7 @@ Browser tooling here has no console access. Runtime errors therefore render into
 
 ## Smart Utils & AI Workflow
 
+- **Global app state**: `src/app/` — `AppStateProvider` (mounted in `main.tsx`) owns the few settings the whole app shares: `theme`, `viewMode`, `sidebarCollapsed`, `searchOpen`, plus the ⌘K/Ctrl+K shortcut. Read them anywhere with `useAppState()` (import from `./app` or `../app`); it throws outside the provider on purpose. Add a setting to `appState.ts` (types) + `AppStateProvider.tsx` (state, DOM mirror, `STORAGE_KEYS` persistence) — never thread a new prop chain. Section-local state (active tab, selected pattern) stays in its section. Persisted settings are written **only on user action**, so an untouched app keeps following `prefers-color-scheme`. `theme` always boots from the `<html data-theme>` attribute the pre-paint script set (storage key `ns-theme` is shared with `index.html`); `viewMode` mirrors to `<html data-view-mode>` and currently gates nothing — it is the audience preset hook for per-persona views.
 - **Class helper**: `src/utils/cn.ts` — zero-dependency classnames merging utility for conditional component styles.
 - **Timer helper**: `src/utils/useTimeout.ts` — `schedule(fn, ms)` / `clear()`; auto-cancels pending timers on unmount. Never fire-and-forget a raw `setTimeout` in a component.
 - **Smart CSS**: `src/utils.css` — ready-to-use helpers (`.dot-canvas`, `.surface-card`, `.surface-sidebar`, `.badge-*`, `.hover-lift`, `.kbd-tag`, `.skeleton-pulse`, `.progress-*`, `.ui-table-*`, `.dialog-*`, `.sheet-*`, `.alert-*`, `.ui-tabs-*`, `.switch-*`).