fix: declare cryptography as a runtime dependency

bcc_core imports cryptography at module load (catalog signature
verification), so it's required to even start the app -- but
requirements.txt listed only PySide6, so a from-source run died with
ModuleNotFoundError: No module named 'cryptography'. The frozen release
builds were unaffected because PyInstaller follows the import, which is
why this never surfaced until someone ran the GUI from source to review
this branch.

Move cryptography into requirements.txt (runtime) and re-label it in
requirements-dev.txt as runtime rather than test-only; the version pin is
unchanged (>=42.0).

Refs #5

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EKwBecy6N83jnqQmw8ezwE

requirements-dev.txt

@@ -1,5 +1,6 @@
 # Runtime (also in requirements.txt)
 PySide6>=6.6
+cryptography>=42.0   # catalog signature verification (bcc_core) + release checksum signing
 
 # Build / packaging
 pyinstaller>=6.0
@@ -8,4 +9,3 @@ pillow>=10.0        # generates icons/app.ico during CI (Windows build)
 # Test / lint
 pytest>=8.0
 ruff>=0.6
-cryptography>=42.0   # release checksum signing (scripts/sign_checksums.py)

requirements.txt

@@ -1 +1,2 @@
 PySide6>=6.6
+cryptography>=42.0   # bcc_core imports it at load (catalog signature verification)