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/en.ts

@@ -615,6 +615,7 @@ export const en = {
   "settings.providerName": "Name (optional)",
   "settings.providerUp": "reachable · {ms} ms",
   "settings.providerDown": "unreachable",
+  "settings.providerAuth": "API key missing or rejected",
   "settings.providerCheck": "Check providers",
   "settings.providerChecking": "Checking…",
   "settings.reloadProviders": "Reload providers & models",