Add auth URL config module with validation (TDD)

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

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});