refactor: migrate to tonutils v2, pytest, pyproject.toml

- tonutils 2.0.0: TonapiClient → ToncenterV3Client, wallet.transfer()
- classes → plain async functions across all utils and methods
- core/: constants.py, cookies.py, exceptions.py extracted
- DEVICE constant in constants.py (single source of truth)
- account/device serialised with json.dumps() in all tx payloads
- tests: unittest → pytest, conftest.py fixtures, 001/002 naming
- pyproject.toml: pytest + ruff config
- min balance check: 0.056 TON

requirements.txt

@@ -1,4 +1,4 @@
-python-dotenv==1.2.1
+python-dotenv==1.2.2
 asyncio==4.0.0
 httpx==0.28.1
-tonutils==0.5.6
+tonutils==2.0.0