fix(i18n): eager-load nl bundle, fix language selector, remove hardcoded backend messages

- Eager-load both en and nl translation bundles at i18n init to eliminate
  the async gap that caused English flash when Dutch was the detected language
- LanguageSwitcher: use i18n.language (synchronous) instead of resolvedLanguage
  (asynchronous) so the visual selection is always correct after switching
- AvailabilityController: remove hardcoded English messages ("System is running
  normally.") from GET /availability/status; return empty string so the frontend
  translations control the display text

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/SlpModularCms.Modules.Availability/Controllers/AvailabilityController.cs

@@ -25,9 +25,7 @@ public class AvailabilityController : ControllerBase
         {
             Status = status.ToString(),
             CheckedAt = DateTimeOffset.UtcNow,
-            Message = status == AvailabilityStatus.Available 
-                ? "System is running normally." 
-                : "System is in maintenance or unavailable."
+            Message = string.Empty,
         });
     }