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>

frontend/src/i18n/LanguageSwitcher.tsx

@@ -20,7 +20,13 @@ import {
 /** Language selector for the topbar; lazy-loads the chosen locale (Q4-B). */
 export function LanguageSwitcher() {
     const { t, i18n } = useTranslation();
-    const current = (i18n.resolvedLanguage ?? 'en') as SupportedLanguage;
+    // Use i18n.language (set synchronously on change) rather than resolvedLanguage
+    // (which can lag when bundles were lazy-loaded). Split on '-' to normalise
+    // region variants like 'nl-NL' → 'nl'.
+    const rawLang = i18n.language?.split('-')[0] ?? 'en';
+    const current = (SUPPORTED_LANGUAGES.includes(rawLang as SupportedLanguage)
+        ? rawLang
+        : 'en') as SupportedLanguage;
 
     return (
         <DropdownMenu>

frontend/src/i18n/config.ts

@@ -2,6 +2,7 @@ import i18n from 'i18next';
 import { initReactI18next } from 'react-i18next';
 import LanguageDetector from 'i18next-browser-languagedetector';
 import enTranslation from './locales/en/translation.json';
+import nlTranslation from './locales/nl/translation.json';
 
 export const SUPPORTED_LANGUAGES = ['en', 'nl'] as const;
 export type SupportedLanguage = (typeof SUPPORTED_LANGUAGES)[number];
@@ -11,25 +12,15 @@ export const LANGUAGE_LABELS: Record<SupportedLanguage, string> = {
     nl: 'Nederlands',
 };
 
-// English (the fallback) is bundled eagerly so the UI never flashes raw keys.
-// Other locales are lazy-loaded on demand (Q4-B / NFR-U1-05).
-const loaded = new Set<string>(['en']);
-
-async function loadLocale(lng: string): Promise<void> {
-    if (loaded.has(lng) || !SUPPORTED_LANGUAGES.includes(lng as SupportedLanguage)) {
-        return;
-    }
-    const module = await import(`./locales/${lng}/translation.json`);
-    i18n.addResourceBundle(lng, 'translation', module.default, true, true);
-    loaded.add(lng);
-}
-
+// Both supported locales are bundled eagerly so the detected language is
+// available immediately on init — no async gap that would cause an English flash.
 void i18n
     .use(LanguageDetector)
     .use(initReactI18next)
     .init({
         resources: {
             en: { translation: enTranslation },
+            nl: { translation: nlTranslation },
         },
         fallbackLng: 'en',
         supportedLngs: SUPPORTED_LANGUAGES as unknown as string[],
@@ -42,12 +33,8 @@ void i18n
         react: { useSuspense: false },
     });
 
-// Ensure the detected language is loaded after init.
-void loadLocale(i18n.resolvedLanguage ?? 'en');
-
-/** Lazy-load the target locale, then switch to it (persisted by the detector). */
+/** Switch to a supported locale and persist the choice via the detector cache. */
 export async function changeLanguage(lng: SupportedLanguage): Promise<void> {
-    await loadLocale(lng);
     await i18n.changeLanguage(lng);
 }