fix(account): preserve existing credentials when saving apikey accounts

When editing an apikey account, the credentials object was built from
scratch, causing fields like tier_id that are not exposed in the UI to
be silently dropped on save. Spread currentCredentials first so unknown
fields are retained, then let the known fields overwrite them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/account/EditAccountModal.vue

@@ -2254,6 +2254,7 @@ const handleSubmit = async () => {
 
       // Always update credentials for apikey type to handle model mapping changes
       const newCredentials: Record<string, unknown> = {
+        ...currentCredentials,
         base_url: newBaseUrl
       }