Refactor imghdr compatibility hack to a dedicated compat module

Moved the imghdr compatibility shim from `src/seedpass/core/password_generation.py`
to a new `src/seedpass/core/compat.py` module. This improves code
cleanliness and separates concerns by centralizing Python version
compatibility logic.

The shim ensures that `imghdr` is available in `sys.modules` (using
`utils.imghdr_stub` as a fallback for Python 3.13+) so that dependencies
like `pgpy` can function correctly.

Verified the change by:
- Confirming `compat.py` correctly populates `sys.modules["imghdr"]`.
- Verifying `password_generation.py` imports correctly and triggers the shim.
- Ensuring syntax correctness via `py_compile`.

Co-authored-by: PR0M3TH3AN <[email protected]>

src/seedpass/core/compat.py

@@ -0,0 +1,11 @@
+"""Compatibility layer for different Python versions."""
+
+import sys
+
+# Ensure the ``imghdr`` module is available for ``pgpy`` on Python 3.13+
+try:  # pragma: no cover - only executed on Python >= 3.13
+    import imghdr  # type: ignore
+except ModuleNotFoundError:  # pragma: no cover - fallback for removed module
+    from utils import imghdr_stub
+
+    sys.modules.setdefault("imghdr", imghdr_stub)

src/seedpass/core/password_generation.py

@@ -35,15 +35,7 @@ from cryptography.hazmat.primitives.asymmetric import ed25519
 from cryptography.hazmat.backends import default_backend
 from bip_utils import Bip39SeedGenerator
 
-# Ensure the ``imghdr`` module is available for ``pgpy`` on Python 3.13+
-try:  # pragma: no cover - only executed on Python >= 3.13
-    import imghdr  # type: ignore
-except ModuleNotFoundError:  # pragma: no cover - fallback for removed module
-    from utils import imghdr_stub as imghdr  # type: ignore
-    import sys
-
-    sys.modules.setdefault("imghdr", imghdr)
-
+from . import compat  # noqa: F401
 from local_bip85.bip85 import BIP85
 
 from constants import (