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;
+}

packages/cli/src/core/initializer.ts

@@ -0,0 +1,45 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { type Config } from '@google/gemini-cli-core';
+import { type LoadedSettings } from '../config/settings.js';
+import { performInitialAuth } from './auth.js';
+import { validateTheme } from './theme.js';
+
+export interface InitializationResult {
+  authError: string | null;
+  themeError: string | null;
+  shouldOpenAuthDialog: boolean;
+  geminiMdFileCount: number;
+}
+
+/**
+ * Orchestrates the application's startup initialization.
+ * This runs BEFORE the React UI is rendered.
+ * @param config The application config.
+ * @param settings The loaded application settings.
+ * @returns The results of the initialization.
+ */
+export async function initializeApp(
+  config: Config,
+  settings: LoadedSettings,
+): Promise<InitializationResult> {
+  const authError = await performInitialAuth(
+    config,
+    settings.merged.security?.auth?.selectedType,
+  );
+  const themeError = validateTheme(settings);
+
+  const shouldOpenAuthDialog =
+    settings.merged.security?.auth?.selectedType === undefined || !!authError;
+
+  return {
+    authError,
+    themeError,
+    shouldOpenAuthDialog,
+    geminiMdFileCount: config.getGeminiMdFileCount(),
+  };
+}

packages/cli/src/core/theme.ts

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+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { themeManager } from '../ui/themes/theme-manager.js';
+import { type LoadedSettings } from '../config/settings.js';
+
+/**
+ * Validates the configured theme.
+ * @param settings The loaded application settings.
+ * @returns An error message if the theme is not found, otherwise null.
+ */
+export function validateTheme(settings: LoadedSettings): string | null {
+  const effectiveTheme = settings.merged.ui?.theme;
+  if (effectiveTheme && !themeManager.findThemeByName(effectiveTheme)) {
+    return `Theme "${effectiveTheme}" not found.`;
+  }
+  return null;
+}