Add a Blazor built-in ErrorBoundary around @Body with a friendly
FluentMessageBar + retry / home buttons. Also hook
NavigationManager.LocationChanged so the boundary resets on any nav
(otherwise the error state persists across NavigateTo and "Zur
Startseite" still shows the failure banner).
U-6 split into U-6a (this) and U-6b (per-handler try/catch to route
expected exceptions to IMessageService — deferred).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
