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/ToastHost.razor

@@ -0,0 +1,41 @@
+@implements IDisposable
+
+@* Shows ToastService's messages at the bottom end of the screen. Placed once, in MainLayout. *@
+@if (Toast.Messages.Count > 0)
+{
+	<DToast class="z-[1000]">
+		@foreach (var message in Toast.Messages)
+		{
+			<DAlert @key="message.Id" Color="ColorOf(message.Severity)" class="max-w-sm">
+				<ChildContent>
+					<span class="break-words">@message.Text</span>
+				</ChildContent>
+				<Actions>
+					<DButton Size="DSize.Xs" Shape="DButtonShape.Circle" Variant="DButtonVariant.Ghost" aria-label="@Localizer["Close"]"
+							 OnClick="() => Toast.Close(message.Id)">
+						<i class="ion-md-close"></i>
+					</DButton>
+				</Actions>
+			</DAlert>
+		}
+	</DToast>
+}
+
+@code {
+	[Inject] ToastService Toast { get; set; }
+	[Inject] CoalescingStringLocalizer Localizer { get; set; }
+
+	protected override void OnInitialized() => Toast.Changed += OnChanged;
+
+	void OnChanged() => _ = InvokeAsync(StateHasChanged);
+
+	static DAlertColor ColorOf(ToastSeverity severity) => severity switch
+	{
+		ToastSeverity.Success => DAlertColor.Success,
+		ToastSeverity.Warning => DAlertColor.Warning,
+		ToastSeverity.Error => DAlertColor.Error,
+		_ => DAlertColor.Info
+	};
+
+	public void Dispose() => Toast.Changed -= OnChanged;
+}