refactor: migrate to tonutils 2.0.0 and restructure codebase

- Migrate from tonutils 0.5.x to 2.0.0 API:
  - ToncenterClient → TonapiClient with NetworkGlobalID
  - tonutils.client → tonutils.clients
  - tonutils.wallet → tonutils.contracts.wallet
  - pub_key.as_hex property, wallet.balance nanotons
  - async with client context manager

- Centralize TON client init into initialize_ton_client() in wallet.py
- Move process_transaction logic into wallet.py, transaction.py re-exports
- Add WALLET_VERSION config (V4R2/V5R1, default V5R1)
- Add WALLET_CLASSES and SUPPORTED_WALLET_VERSIONS to constants.py
- Restore balance check before broadcasting
- Wait for seqno confirmation after transfer (120×2s) to prevent duplicate seqno

- Fix Fragment hash fetching: use browser navigation headers
- Remove accept-encoding from BASE_HEADERS (httpx handles decompression)
- Add cookies.example.json template
- Add cookies.json to .gitignore

tests/001_test_decode.py

@@ -1,5 +1,6 @@
 """Tests for clean_decode() — BOC-encoded Fragment payloads decode to
 human-readable UTF-8 with the Telegram label and Ref# intact."""
+
 import re
 
 import pytest

tests/002_test_hash.py

@@ -1,5 +1,6 @@
 """Tests for get_fragment_hash() — fetches a valid lowercase hex hash
 from the fragment.com/stars/buy page source."""
+
 import re
 
 import pytest