docs: add integration testing documentation

- Create docs/TESTING.md with instructions for running Playwright tests.
- Document configuration, environment variables, and troubleshooting tips.

docs/TESTING.md

@@ -0,0 +1,88 @@
+# Integration Testing
+
+Hördle uses [Playwright](https://playwright.dev/) for end-to-end (E2E) integration testing. These tests ensure that critical flows like gameplay, authentication, and admin management function correctly across different browsers.
+
+## Prerequisites
+
+Ensure you have the Playwright browsers installed:
+
+```bash
+npx playwright install
+```
+
+## Running Tests
+
+### Headless Mode (CI/CLI)
+
+To run all tests in headless mode (Chromium, Firefox, WebKit):
+
+```bash
+npm run test:e2e
+```
+
+### UI Mode (Interactive)
+
+To run tests with a UI to inspect traces and watch execution:
+
+```bash
+npm run test:e2e:ui
+```
+
+### Specific Test File
+
+To run a specific test file:
+
+```bash
+npx playwright test tests/gameplay.spec.ts
+```
+
+### Specific Project (Browser)
+
+To run tests only on a specific browser (e.g., Chromium):
+
+```bash
+npx playwright test --project=chromium
+```
+
+## Configuration
+
+The Playwright configuration is located in `playwright.config.ts`. It sets up the base URL (default: `http://localhost:3000`) and the web server command to start the app if it's not running.
+
+### Environment Variables
+
+*   **`ADMIN_PASSWORD`**: The tests assume the admin password is `'admin123'`. 
+    *   In `app/api/admin/login/route.ts`, the login logic has been enhanced to check if `ADMIN_PASSWORD` is a bcrypt hash (starts with `$2b$`) or plain text. 
+    *   For local testing, you can set `ADMIN_PASSWORD="admin123"` in your `.env` or `.env.local` (though the default fallback in the code also handles this).
+*   **Curator Credentials**: The mock Curator login page (`app/[locale]/curator/page.tsx`) mocks validation for testing.
+    *   Username: `elpatron`
+    *   Password: `surf&4033`
+
+## Test Structure
+
+Tests are located in the `tests/` directory:
+
+*   **`auth.spec.ts`**: Verifies public access and admin login flows.
+*   **`admin.spec.ts`**: Checks the Admin Dashboard, including access protection and visibility of sections (Dashboard, Daily Puzzles, etc.).
+*   **`curator.spec.ts`**: Verifies the Curator login form and authentication flows (valid/invalid credentials).
+*   **`gameplay.spec.ts`**: Tests the core game loop: loading the game, playing audio, interaction with the prompt, and submitting a guess.
+
+## Troubleshooting & Known Issues
+
+### Next.js Development Overlay (`nextjs-portal`)
+
+In development mode (`npm run dev`), Next.js injects an overlay (`<nextjs-portal>`) for hot reloading feedback. This overlay can sometimes intercept clicks intended for UI elements, causing tests to fail with "element is not clickable" or timeout errors.
+
+**Solution:** 
+We inject a CSS style in the `beforeEach` hook of our tests to hide this overlay:
+
+```typescript
+test.beforeEach(async ({ page }) => {
+    await page.addStyleTag({ content: 'nextjs-portal, #nextjs-dev-overlay, [data-nextjs-dev-overlay] { display: none !important; }' });
+});
+```
+
+### WebKit (Safari) Stability
+
+WebKit can be slower or more sensitive to timing in some environments. If tests fail on WebKit but pass on other browsers:
+1.  Try increasing the timeout in `playwright.config.ts`.
+2.  Use `await page.waitForTimeout(500)` or specific assertions like `await expect(page).toHaveURL(...)` to allow for transition times, as implemented in `tests/admin.spec.ts`.