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

main.py

@@ -29,7 +29,9 @@ async def buy_premium_example():
 
     if result["success"]:
         data = result["data"]
-        logger.info(f"Premium purchase successful: {data['months']} months sent to {data['username']}")
+        logger.info(
+            f"Premium purchase successful: {data['months']} months sent to {data['username']}"
+        )
         logger.info(f"Transaction ID: {data['transaction_id']}")
     else:
         logger.error(f"Premium purchase failed: {result['error']}")