feat(models): auto-derive id from model name; actionable probe auth errors

- The model id is now auto-filled from the model name as you type, and if left
  blank it's derived on save (slug of the last path segment, e.g.
  "qwen/qwen3.7-max" → "qwen3-7-max") — saving no longer errors on a blank id.
- probe/test auth failures (401 / "User not found" / "No auth") now append a
  hint to check the API key (for OpenRouter: use an inference key, not a
  provisioning/management key).

web/js/i18n/en.js

@@ -129,6 +129,7 @@ export default {
   "models.form.secretMissing": "not set",
   "models.form.secretPlaceholder": "set or replace key (write-only)",
   "models.deleteConfirm": "Delete model \"{id}\"?",
+  "models.form.idOrModel": "Enter an id or a model name",
   "models.form.apiKey": "API Key",
   "models.form.apiKeyPlaceholder": "paste your API key",
   "models.form.apiKeyNotNeeded": "not required (local server)",

web/js/i18n/ru.js

@@ -128,6 +128,7 @@ export default {
   "models.form.secretMissing": "не задан",
   "models.form.secretPlaceholder": "задать или заменить ключ (только запись)",
   "models.deleteConfirm": "Удалить модель \"{id}\"?",
+  "models.form.idOrModel": "Укажите id или имя модели",
   "models.form.apiKey": "API-ключ",
   "models.form.apiKeyPlaceholder": "вставьте API-ключ",
   "models.form.apiKeyNotNeeded": "не нужен (локальный сервер)",