fix(login): address PR review feedback

- Use apiClient instead of raw fetch() in configApi.getPublicConfig
- Validate DEFAULT_LANGUAGE against supported codes on server startup
- Log warning instead of silently swallowing fetch errors in LoginPage
- Case-insensitive browser language matching in detectBrowserLanguage
- Guard against undefined navigator in detectBrowserLanguage
- Validate language code in setLanguageTransient before applying
- Import directly from TranslationContext instead of barrel index

client/src/api/client.ts

@@ -380,7 +380,7 @@ export const weatherApi = {
 
 export const configApi = {
   getPublicConfig: (): Promise<{ defaultLanguage: string }> =>
-    fetch('/api/config').then(r => r.json()),
+    apiClient.get('/config').then(r => r.data),
 }
 
 export const settingsApi = {