docs: note Amplify preBuild runs unit tests

Author: Partha-dev01

docs/SETUP_GUIDE.md

@@ -43,8 +43,8 @@ Environment variables are configured in the Amplify Console:
 ## Testing
 
 ```bash
-npm run test:unit      # Vitest unit tests (68 tests)
-npx playwright test    # Playwright E2E tests (97 tests)
+npm run test:unit      # Vitest unit tests (68 tests, also runs in Amplify preBuild)
+npx playwright test    # Playwright E2E tests (~97 tests)
 npm run lint           # ESLint
 npm run type-check     # TypeScript
 ```