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/utils/__init__.py

@@ -0,0 +1,16 @@
+from fragmentapi.utils.client import (
+    execute_transaction_request,
+    get_fragment_hash,
+    parse_json_response,
+)
+from fragmentapi.utils.decoder import clean_decode
+from fragmentapi.utils.wallet import get_account_info, process_transaction
+
+__all__ = [
+    "clean_decode",
+    "execute_transaction_request",
+    "get_account_info",
+    "get_fragment_hash",
+    "parse_json_response",
+    "process_transaction",
+]