- Create useGlobalErrorHandler composable that wires up three
  global error-catching surfaces:
    1. app.config.errorHandler — Vue runtime errors (render,
       watchers, lifecycle hooks, event handlers).
    2. window 'unhandledrejection' — rejected promises without catch.
    3. window 'error' — synchronous JS runtime errors.
- Create AppErrorBoundary component wrapping RouterView to catch
  Vue errors in child components and show fallback UI (AppErrorState
  with Retry) instead of a blank screen.
- Apply the boundary in App.vue so every routed page is protected.

All 126 Vitest tests pass.