refactor: restructure as installable PyPI package

- Rename app/ → fragmentapi/ for proper package naming
- Add FragmentClient class with gift_premium, gift_stars, topup_ton methods
- Restructure core/ → types/ (exceptions, results, constants)
- Merge utils/hash.py into utils/client.py
- Replace _version.py with importlib.metadata
- Add input validation with min/max bounds
- Add unit tests: decode, client init/cookies
- Rename 002_test_hash → 003_test_hash
- Clean up pyproject.toml: pin deps, production classifiers

fragmentapi/__init__.py

@@ -0,0 +1,44 @@
+# Copyright (c) 2025 bohd4nx
+#
+# This source code is licensed under the MIT License found in the
+# LICENSE file in the root directory of this source tree.
+
+from fragmentapi.client import FragmentClient
+from fragmentapi.types import (
+    AdsTopupResult,
+    ClientError,
+    ConfigError,
+    CookiesError,
+    FragmentAPIError,
+    FragmentError,
+    HashFetchError,
+    OperationError,
+    PremiumResult,
+    RequestError,
+    StarsResult,
+    TransactionError,
+    UnexpectedError,
+    UserNotFoundError,
+    VerificationError,
+    WalletError,
+)
+
+__all__ = [
+    "FragmentClient",
+    "AdsTopupResult",
+    "PremiumResult",
+    "StarsResult",
+    "ClientError",
+    "ConfigError",
+    "CookiesError",
+    "FragmentAPIError",
+    "FragmentError",
+    "HashFetchError",
+    "OperationError",
+    "RequestError",
+    "TransactionError",
+    "UnexpectedError",
+    "UserNotFoundError",
+    "VerificationError",
+    "WalletError",
+]