Provider status: tell rejected API keys apart from unreachable hosts

A 404/405 on the model list counts as reachable (endpoint without a list).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/i18n/de.ts

@@ -616,6 +616,7 @@ export const de: Messages = {
   "settings.providerName": "Name (optional)",
   "settings.providerUp": "erreichbar · {ms} ms",
   "settings.providerDown": "nicht erreichbar",
+  "settings.providerAuth": "API-Key fehlt oder ungültig",
   "settings.providerCheck": "Provider prüfen",
   "settings.providerChecking": "Prüfe…",
   "settings.reloadProviders": "Provider & Modelle neu laden",