House rules: numbers through one formatter, no made-up server errors, and the rules fail the build

- Every number shown on a page goes through NumbersExtensions: counts,
  percentages, the nerd readouts and the values written into a style.
- A failure the client builds itself (offline, timeout, retries run out, an
  answer it can't read) is a 400 that names its kind; only a server's own 5XX
  passes through.
- Login, sign-up, recovery and password confirmations leave autocomplete to
  the browser, so it can offer the saved login.
- The Daisy components start with empty lists and attributes, and a
  decorative image still renders alt="".
- Arasaka.HouseRules.Analyzers checks all of this in every build, together
  with PrivaPub.ClientModels.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W4YtEJrgxG12Pc39Fai8Bd

Services/InstanceAdminService.cs

@@ -65,7 +65,7 @@ namespace decePubClient.Services
 			if (!current.IsSuccessStatusCode)
 				return await current.Content.ReadWebResult(current.StatusCode, localizer, cancellationToken);
 			if (await current.DefaultReadFromJsonAsync<ViewServerSettings>(cancellationToken) is not { } latest)
-				return new WebResult().Invalidate(localizer["The server sent an invalid response."], (int)HttpStatusCode.BadGateway);
+				return new WebResult().Invalidate(localizer["The server sent an invalid response."], (int)HttpStatusCode.BadRequest);
 
 			var form = FormOf(latest);
 			change(form);