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/methods/__init__.py

@@ -0,0 +1,5 @@
+from fragmentapi.methods.premium import gift_premium
+from fragmentapi.methods.stars import gift_stars
+from fragmentapi.methods.ton import topup_ton
+
+__all__ = ["gift_premium", "gift_stars", "topup_ton"]