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

Shared/NavMenu.razor

@@ -9,19 +9,19 @@
 		<div class="absolute bg-base-100 md:relative top-full md:top-auto mt-2 w-52 md:w-full overflow-y-auto rounded-lg neo-raised neo-sm md:shadow-none! z-50 @menuToggle md:block">
 			<div class="flex flex-col gap-3 p-3 md:p-4">
 
-				<div class="flex gap-3 p-3 md:p-4 neo-raised neo-xs rounded-xl bg-cover bg-no-repeat bg-right" style="background-image:linear-gradient(to left, var(--color-base-100) 55%, transparent), url('@(CascadingState.User?.BackgroundUrl)');">
-					<a class="block h-8 w-8 md:h-12 md:w-12 flex-none" href="@CascadingState.User?.ProfileUrl" title="@CascadingState.User?.UserName">
-						<img alt="@CascadingState.User?.UserName" class="h-8 w-8 md:h-12 md:w-12 object-cover rounded-full neo-raised neo-2xs" src="@(CascadingState.User?.PictureUrl ?? "/imgs/icon-192.png")" />
+				<div class="flex gap-3 p-3 md:p-4 neo-raised neo-xs rounded-xl bg-cover bg-no-repeat bg-right" style="background-image:linear-gradient(to left, var(--color-base-100) 55%, transparent), url('@(CascadingState.User.BackgroundUrl)');">
+					<a class="block h-8 w-8 md:h-12 md:w-12 flex-none" href="@CascadingState.User.ProfileUrl" title="@CascadingState.User.UserName">
+						<img alt="@CascadingState.User.UserName" class="h-8 w-8 md:h-12 md:w-12 object-cover rounded-full neo-raised neo-2xs" src="@(CascadingState.User.PictureUrl ?? "/imgs/icon-192.png")" />
 					</a>
 					<div class="flex flex-col w-full min-w-0 gap-2">
 						<p class="shrink truncate">
-							<b title="@CascadingState.User?.DisplayName">
-								@CascadingState.User?.DisplayName
+							<b title="@CascadingState.User.DisplayName">
+								@CascadingState.User.DisplayName
 							</b>
 						</p>
 						<p class="flex-1 min-w-6 truncate text-xs">
-							<a class="underline" href="@CascadingState.User?.ProfileUrl" title="@CascadingState.User?.UserName">
-								@CascadingState.User?.UserName
+							<a class="underline" href="@CascadingState.User.ProfileUrl" title="@CascadingState.User.UserName">
+								@CascadingState.User.UserName
 							</a>
 						</p>
 					</div>
@@ -94,21 +94,19 @@
 
 	<div class="md:hidden">
 		<a class="block rounded-full neo-raised neo-2xs" href="settings">
-			<img alt="@CascadingState.User?.UserName" class="object-cover w-[30px] h-[30px] rounded-full"
-					 src="@CascadingState.User?.PictureUrl">
+			<img alt="@CascadingState.User.UserName" class="object-cover w-[30px] h-[30px] rounded-full"
+					 src="@CascadingState.User.PictureUrl">
 		</a>
 	</div>
 
 </nav>
 
 @code {
-	[CascadingParameter] CascadingState CascadingState { get; set; }
 	string menuToggle = "hidden";
 
-	PageSettings Settings => CascadingState.PublicCacheData?.PageSettings ?? new();
 	// The toggle and the slider work on the variant that is showing; with system theming that is the OS's choice.
 	bool IsDark => CascadingState.Status.IsDarkTheme();
-	short ThemeIndexColour => IsDark ? Settings.DarkThemeIndexColour : Settings.LightThemeIndexColour;
+	short ThemeIndexColour => IsDark ? CascadingState.PublicCacheData.PageSettings.DarkThemeIndexColour : CascadingState.PublicCacheData.PageSettings.LightThemeIndexColour;
 
 	protected override void OnInitialized()
 	{