Adopt b2b.next's client patterns: services, page state, feedback

Ported from the owner's b2b.next and adapted from MudBlazor to the daisyUI
components:
- LocalizableComponentBase provides CascadingState, Navigation, Toast,
  IsLoading/IsDefaultDisabled, WhenOnline/StartOnlinePolling, and after-render
  jobs that actually run.
- CascadingState starts with initialised state and polls the online status
  with a PeriodicTimer. It reports errors through ILoggingService plus a toast,
  and disposes what it holds.
- New pieces:
  - ToastService + ToastHost, the snackbar, with AddResponseError/Success/
    Warning/Exception in SUtility;
  - ErrorViewer, which shows the validation messages plus AdditionalError(s);
  - LoggingService, which writes to the console and the IndexedDB client log.
- HttpService is rebuilt and registered:
  - auth levels Get/Post/Delete, *Anon and *TotallyAnon;
  - retry with backoff;
  - classified failures (FailureCodes) instead of exceptions;
  - HTTP 410 on cancel;
  - the "default" client.
  ReadWebResult and DefaultReadFromJsonAsync read its responses.
- ClientJsonContext is source-generated, and SUtility.DefaultSerializer and
  Blazored.LocalStorage use it.
- WebResult gains a C# 14 IsInvalid extension property.

The code follows the imported rules:
- Every service and async helper takes a CancellationToken last.
- Primary constructors replace constructor-plus-fields.
- MessagesService and IStorage return Task<WebResult> / Task<List<T>>.
  MessagesService is still mock, building its results in memory; the seed
  feed and thread moved to Faker.
- Index and ExpandMessage follow the page pattern: an offline guard with
  WhenOnline, LoadData, and early returns with toasts on the results they used
  to ignore. ExpandMessage reloads on route change and shows "not found"
  instead of a null message.
- Model and state collections are [], and the fake identity defaults moved
  out of the models.
- Components:
  - no ?. chains over initialised state;
  - [Inject] instead of @inject;
  - IsDefaultDisabled on the actions;
  - autocomplete on every text input;
  - Localizer[key, args] instead of string.Format;
  - the localizer no longer leaks its inner _pLocalizer.
- Settings → General gains a language picker (English/Italiano), and
  SetDefaultCulture limits the culture to those two.

Bugs fixed along the way:
- A compose with untouched (null) content passed the empty check.
- File errors were joined with <br/> and rendered as markup.
- ClientLogs.ErrorMessage() threw on warnings.
- CoalescingStringLocalizer[key, args] showed a raw "{0}" for missing keys.
- AppStatusService exposed a call to a JS function that doesn't exist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Components/ErrorViewer.razor

@@ -0,0 +1,58 @@
+@implements IDisposable
+
+@* In-page errors (b2b.next's WidmannErrorViewer): the validation messages of the cascading EditContext, then
+   AdditionalError (one message) and AdditionalErrors (a list; never join a list into one string), one <li> each.
+   Renders nothing when there is nothing to show. Messages are text, not markup: they can contain file names. *@
+@if (HasContent)
+{
+	<DAlert Color="@(Severity is ErrorViewerSeverity.Warning ? DAlertColor.Warning : DAlertColor.Error)" Variant="DAlertVariant.Soft"
+			class="@Cx("text-sm", ContainerClass)">
+		<ul class="list-inside list-disc">
+			@foreach (var message in Messages)
+			{
+				<li>@message</li>
+			}
+		</ul>
+	</DAlert>
+}
+
+@code {
+	[CascadingParameter] EditContext EditContext { get; set; }
+	[Parameter] public string ContainerClass { get; set; }
+	[Parameter] public ErrorViewerSeverity Severity { get; set; }
+	[Parameter] public string AdditionalError { get; set; }
+	[Parameter] public IEnumerable<string> AdditionalErrors { get; set; } = [];
+
+	EditContext subscribedEditContext;
+
+	IEnumerable<string> Messages =>
+		(EditContext?.GetValidationMessages() ?? [])
+		.Append(AdditionalError)
+		.Concat(AdditionalErrors)
+		.Where(message => !string.IsNullOrEmpty(message));
+
+	bool HasContent => Messages.Any();
+
+	static string Cx(params string[] classes) => DCx.Join(classes);
+
+	protected override void OnParametersSet()
+	{
+		if (EditContext == subscribedEditContext)
+			return;
+
+		Unsubscribe();
+		subscribedEditContext = EditContext;
+		if (subscribedEditContext is not null)
+			subscribedEditContext.OnValidationStateChanged += OnValidationStateChanged;
+	}
+
+	void OnValidationStateChanged(object sender, ValidationStateChangedEventArgs e) => _ = InvokeAsync(StateHasChanged);
+
+	void Unsubscribe()
+	{
+		if (subscribedEditContext is not null)
+			subscribedEditContext.OnValidationStateChanged -= OnValidationStateChanged;
+	}
+
+	public void Dispose() => Unsubscribe();
+}