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

Services/ToastService.cs

@@ -0,0 +1,55 @@
+namespace decePubClient.Services;
+
+public enum ToastSeverity
+{
+	Info,
+	Success,
+	Warning,
+	Error
+}
+
+public sealed record ToastMessage(Guid Id, ToastSeverity Severity, string Text);
+
+/// <summary>
+/// The app's snackbar: components add messages (usually through the SUtility extensions AddResponseError,
+/// AddResponseSuccess, AddWarning, AddException) and Components/ToastHost.razor, placed once in MainLayout, shows them.
+/// A message already on screen isn't added twice, and at most MaxMessages stay, oldest dropped first.
+/// </summary>
+public class ToastService
+{
+	const int MaxMessages = 12;
+
+	readonly List<ToastMessage> messages = [];
+
+	public event Action Changed;
+
+	public IReadOnlyList<ToastMessage> Messages => messages;
+
+	/// <summary>Shows a message; with closeAfter it closes itself, otherwise it stays until the user closes it.</summary>
+	public void Add(ToastSeverity severity, string text, TimeSpan? closeAfter = null)
+	{
+		if (string.IsNullOrWhiteSpace(text) || messages.Any(m => m.Severity == severity && m.Text == text))
+			return;
+
+		var message = new ToastMessage(Guid.NewGuid(), severity, text);
+		messages.Add(message);
+		if (messages.Count > MaxMessages)
+			messages.RemoveAt(0);
+		Changed?.Invoke();
+
+		if (closeAfter is { } delay)
+			_ = CloseLater(message.Id, delay);
+	}
+
+	public void Close(Guid id)
+	{
+		if (messages.RemoveAll(m => m.Id == id) > 0)
+			Changed?.Invoke();
+	}
+
+	async Task CloseLater(Guid id, TimeSpan delay, CancellationToken cancellationToken = default)
+	{
+		await Task.Delay(delay, cancellationToken);
+		Close(id);
+	}
+}