security: move credentials to .env file

- Remove hardcoded logins/passwords from conftest.py
- Load credentials via python-dotenv from .env (excluded from git)
- Add BASE_URL env variable for easy environment switching
- Add .env.example template for new contributors
- Add validation: clear error message if .env is not filled
- Add python-dotenv to requirements.txt

requirements.txt

@@ -1,5 +1,6 @@
-playwright
-pytest
-pytest-playwright
-pytest-xdist
-pytest-html==3.2.0
+playwright
+pytest
+pytest-playwright
+pytest-xdist
+pytest-html==3.2.0
+python-dotenv