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

Pages/Settings.razor

@@ -15,23 +15,32 @@
 			<ChildContent>
 				@* Every theme setting; NavMenu's toggle and slider are shortcuts to the dark and hue ones. *@
 				<div class="flex flex-col gap-4 w-full p-3 md:p-4">
-					<DToggle LabelText="@Localizer["Follow the system's light or dark setting"]" Value="Theme.PreferSystemTheming"
-							 ValueChanged="v => Update(s => s.PreferSystemTheming = v)" ValueExpression="() => Theme.PreferSystemTheming" />
+					<DToggle LabelText="@Localizer["Follow the system's light or dark setting"]" Value="CascadingState.PublicCacheData.PageSettings.PreferSystemTheming"
+							 ValueChanged="v => Update(s => s.PreferSystemTheming = v)" ValueExpression="() => CascadingState.PublicCacheData.PageSettings.PreferSystemTheming" />
 					<DToggle LabelText="@Localizer["Dark mode"]" Value="IsDark"
 							 ValueChanged="v => Update(s => s.SetDarkMode(v))" ValueExpression="() => IsDark" />
 					<DToggle LabelText="@Localizer["Grey"]" Value="IsGray"
 							 ValueChanged="v => Update(s => s.SetGray(v, IsDark))" ValueExpression="() => IsGray" />
+					<label class="flex flex-col gap-2 max-w-md">
+						<span class="text-sm">@Localizer["Language"]</span>
+						@* Language names stay in their own language. Changing it reloads the app with the new culture. *@
+						<DSelect TValue="string" class="w-full" Value="CascadingState.PublicCacheData.PageSettings.CurrentLanguageCode"
+								 ValueChanged="ChangeLanguage" ValueExpression="() => CascadingState.PublicCacheData.PageSettings.CurrentLanguageCode">
+							<option value="en">English</option>
+							<option value="it">Italiano</option>
+						</DSelect>
+					</label>
 					<label class="flex flex-col gap-2 max-w-md">
 						<span class="text-sm">@Localizer["Light theme colour"]</span>
 						<DRange TValue="short" class="neo-hue-track w-full" Size="DSize.Xs" Min="0" Max="359" Step="1"
-								Value="Theme.LightThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(false, v))"
-								ValueExpression="() => Theme.LightThemeIndexColour" />
+								Value="CascadingState.PublicCacheData.PageSettings.LightThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(false, v))"
+								ValueExpression="() => CascadingState.PublicCacheData.PageSettings.LightThemeIndexColour" />
 					</label>
 					<label class="flex flex-col gap-2 max-w-md">
 						<span class="text-sm">@Localizer["Dark theme colour"]</span>
 						<DRange TValue="short" class="neo-hue-track w-full" Size="DSize.Xs" Min="0" Max="359" Step="1"
-								Value="Theme.DarkThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(true, v))"
-								ValueExpression="() => Theme.DarkThemeIndexColour" />
+								Value="CascadingState.PublicCacheData.PageSettings.DarkThemeIndexColour" ValueChanged="v => Update(s => s.SetHue(true, v))"
+								ValueExpression="() => CascadingState.PublicCacheData.PageSettings.DarkThemeIndexColour" />
 					</label>
 				</div>
 			</ChildContent>
@@ -87,11 +96,15 @@
 </section>
 
 @code {
-	[CascadingParameter] CascadingState CascadingState { get; set; }
 
-	PageSettings Theme => CascadingState.PublicCacheData?.PageSettings ?? new();
 	bool IsDark => CascadingState.Status.IsDarkTheme();
-	bool IsGray => Theme.IsGray();
+	bool IsGray => CascadingState.PublicCacheData.PageSettings.IsGray();
 
 	Task Update(Action<PageSettings> change) => CascadingState.UpdatePageSettings(change).AsTask();
+
+	async Task ChangeLanguage(string languageCode)
+	{
+		await Update(settings => settings.CurrentLanguageCode = languageCode);
+		Navigation.NavigateTo(Navigation.Uri, forceLoad: true);
+	}
 }