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

app/core/constants.py

@@ -1,34 +1,44 @@
 import json
 
+from tonutils.contracts.wallet import WalletV4R2, WalletV5R1
+
+# Supported TON wallet contract versions
+SUPPORTED_WALLET_VERSIONS: set[str] = {"V4R2", "V5R1"}
+
+# Wallet class map — used to resolve the correct contract from WALLET_VERSION
+WALLET_CLASSES: dict[str, type] = {"V4R2": WalletV4R2, "V5R1": WalletV5R1}
+
 # Fragment page URLs
-STARS_PAGE:   str = "https://fragment.com/stars/buy"
+STARS_PAGE: str = "https://fragment.com/stars/buy"
 PREMIUM_PAGE: str = "https://fragment.com/premium/gift"
-ADS_PAGE:     str = "https://fragment.com/ads/topup"
+ADS_PAGE: str = "https://fragment.com/ads/topup"
 
 # Tonkeeper device fingerprint — serialized once, reused in every tx_data payload.
-DEVICE: str = json.dumps({
-    "platform":           "iphone",
-    "appName":            "Tonkeeper",
-    "appVersion":         "5.5.2",
-    "maxProtocolVersion": 2,
-    "features": [
-        "SendTransaction",
-        {"name": "SendTransaction", "maxMessages": 255},
-        {"name": "SignData", "types": ["text", "binary", "cell"]},
-    ],
-})
+DEVICE: str = json.dumps(
+    {
+        "platform": "iphone",
+        "appName": "Tonkeeper",
+        "appVersion": "5.5.2",
+        "maxProtocolVersion": 2,
+        "features": [
+            "SendTransaction",
+            {"name": "SendTransaction", "maxMessages": 255},
+            {"name": "SignData", "types": ["text", "binary", "cell"]},
+        ],
+    }
+)
 
 # Base HTTP headers — shared across all Fragment API requests.
 # Each method merges these with its own "referer" and "x-aj-referer".
 BASE_HEADERS: dict[str, str] = {
-    "accept":           "application/json, text/javascript, */*; q=0.01",
-    "accept-language":  "en-US,en;q=0.9,uk;q=0.8,ru;q=0.7",
-    "content-type":     "application/x-www-form-urlencoded; charset=UTF-8",
-    "origin":           "https://fragment.com",
-    "priority":         "u=1, i",
-    "sec-fetch-dest":   "empty",
-    "sec-fetch-mode":   "cors",
-    "sec-fetch-site":   "same-origin",
+    "accept": "application/json, text/javascript, */*; q=0.01",
+    "accept-language": "en-US,en;q=0.9,uk;q=0.8,ru;q=0.7",
+    "content-type": "application/x-www-form-urlencoded; charset=UTF-8",
+    "origin": "https://fragment.com",
+    "priority": "u=1, i",
+    "sec-fetch-dest": "empty",
+    "sec-fetch-mode": "cors",
+    "sec-fetch-site": "same-origin",
     "user-agent": (
         "Mozilla/5.0 (iPhone; CPU iPhone OS 18_5 like Mac OS X) "
         "AppleWebKit/605.1.15 (KHTML, like Gecko) Version/18.5 Mobile/15E148 Safari/604.1"