PR U-6a: MainLayout wraps @Body in an ErrorBoundary

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>

Schaad.Accounting.UI/Components/Layout/MainLayout.razor

@@ -1,4 +1,6 @@
 @inherits LayoutComponentBase
+@inject NavigationManager Navigation
+@implements IDisposable
 
 <FluentLayout>
     <FluentHeader Class="hidePrint">
@@ -9,7 +11,24 @@
         <FluentBodyContent Class="body-content">
             <FluentMessageBarProvider Section="MESSAGES_TOP" @rendermode="InteractiveServer" />
             <div class="content">
-                @Body
+                <ErrorBoundary @ref="errorBoundary">
+                    <ChildContent>
+                        @Body
+                    </ChildContent>
+                    <ErrorContent>
+                        <FluentStack Orientation="Orientation.Vertical" Style="padding: 2rem; max-width: 640px;">
+                            <FluentMessageBar Intent="MessageIntent.Error"
+                                              Title="Ein Fehler ist aufgetreten"
+                                              AllowDismiss="false">
+                                Diese Seite konnte nicht angezeigt werden. Details stehen im Server-Log.
+                            </FluentMessageBar>
+                            <FluentStack Orientation="Orientation.Horizontal">
+                                <FluentButton Appearance="Appearance.Neutral" OnClick="Recover">Neu versuchen</FluentButton>
+                                <FluentAnchor Href="/" Appearance="Appearance.Neutral">Zur Startseite</FluentAnchor>
+                            </FluentStack>
+                        </FluentStack>
+                    </ErrorContent>
+                </ErrorBoundary>
             </div>
         </FluentBodyContent>
     </FluentStack>
@@ -28,4 +47,23 @@
     An unhandled error has occurred.
     <a href="." class="reload">Reload</a>
     <span class="dismiss">🗙</span>
-</div>
+</div>
+
+@code {
+    private ErrorBoundary? errorBoundary;
+
+    protected override void OnInitialized()
+    {
+        // ErrorBoundary state persists across NavigationManager.NavigateTo, so the
+        // error page would linger even after the user clicked "Zur Startseite".
+        // Reset on any location change.
+        Navigation.LocationChanged += OnLocationChanged;
+    }
+
+    private void OnLocationChanged(object? sender, Microsoft.AspNetCore.Components.Routing.LocationChangedEventArgs e)
+        => errorBoundary?.Recover();
+
+    private void Recover() => errorBoundary?.Recover();
+
+    public void Dispose() => Navigation.LocationChanged -= OnLocationChanged;
+}

UI_IMPROVEMENT_PLAN.md

@@ -165,7 +165,8 @@ Split into two PRs on delivery because the header path touches ~15 pages.
 ### Phase 3 — Loading and error UX consistency
 
 - **U-5** Shared `<LoadingWhen Data="@x">` component with a consistent skeleton visual. Adopt across every page currently using ad-hoc `@if (x is null) { <p>Lädt…</p> return; }`.
-- **U-6** Wrap each page (or the layout's `@Body`) in `<ErrorBoundary>` with a friendly retry UI. Route `SaveTransactionAsync` / `DeleteAccountAsync` etc. exceptions to `IMessageService` instead of bubbling.
+- **U-6a** Wrap `MainLayout`'s `@Body` in `<ErrorBoundary>` with a friendly retry UI. Reset on any `NavigationManager.LocationChanged` so the error state doesn't stick after the user clicks "Zur Startseite".
+- **U-6b** Route expected `SaveTransactionAsync` / `DeleteAccountAsync` / etc. exceptions to `IMessageService` — try/catch in each of the ~15 event handlers with a shared error-formatting helper.
 
 ### Phase 4 — Feedback + polish for mutations