feat(ui): Introduce useUI Hook and UIContext (#5488)

Co-authored-by: Jacob Richman <jacob314@gmail.com>

packages/cli/src/core/auth.ts

@@ -0,0 +1,36 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import {
+  type AuthType,
+  type Config,
+  getErrorMessage,
+} from '@google/gemini-cli-core';
+
+/**
+ * Handles the initial authentication flow.
+ * @param config The application config.
+ * @param authType The selected auth type.
+ * @returns An error message if authentication fails, otherwise null.
+ */
+export async function performInitialAuth(
+  config: Config,
+  authType: AuthType | undefined,
+): Promise<string | null> {
+  if (!authType) {
+    return null;
+  }
+
+  try {
+    await config.refreshAuth(authType);
+    // The console.log is intentionally left out here.
+    // We can add a dedicated startup message later if needed.
+  } catch (e) {
+    return `Failed to login. Message: ${getErrorMessage(e)}`;
+  }
+
+  return null;
+}