fix(addin-host): treat API key alone as configured (don't require username)

After the previous fix made Username optional in the Settings dialog and
the credential store, TryInitializeCrm still bailed out when Username
was blank — leaving the add-in in "not configured" state even though
the user had saved a valid API key.

Only ApiKey is required now. The check matches the new contract used by
EspoCrmClient (X-Api-Key alone is sufficient for API-User credentials).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/OutlookAddin/Services/AddInHost.cs

@@ -284,9 +284,9 @@ namespace OutlookAddin.Services
             DisposeCrmStack();
 
             var creds = CredentialStore.Load();
-            if (creds == null || string.IsNullOrWhiteSpace(creds.Username) || string.IsNullOrWhiteSpace(creds.ApiKey))
+            if (creds == null || string.IsNullOrWhiteSpace(creds.ApiKey))
             {
-                Logger.Information("AddInHost: no stored credentials — CRM features disabled until first-run setup");
+                Logger.Information("AddInHost: no stored API key — CRM features disabled until first-run setup");
                 CrmClientChanged?.Invoke(this, EventArgs.Empty);
                 return;
             }