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/decoder.py

@@ -0,0 +1,20 @@
+import base64
+
+from pytoniq_core import Cell
+
+from fragmentapi.types import RequestError
+
+
+def clean_decode(payload: str) -> str:
+    s = payload.strip()
+    if not s:
+        return ""
+    s += "=" * (-len(s) % 4)
+    try:
+        boc = base64.b64decode(s)
+        cell = Cell.one_from_boc(boc)
+        sl = cell.begin_parse()
+        sl.load_uint(32)  # op code — always 0 for text comment
+        return sl.load_snake_string().strip()
+    except Exception as exc:
+        raise RequestError(RequestError.UNPARSEABLE.format(context="payload decode", exc=exc)) from exc