Closes #113.

This keeps the PR focused on the macOS custom-shortcut capture path and the resulting UI wording:

• move Qt key and modifier interpretation into the backend so captured Command and Control chords normalize to stored super and ctrl correctly on macOS
• accept raw Qt enum objects from QML and switch the key handler to an explicit function form to avoid the runtime enum coercion issue
• keep Super as the canonical displayed label while still accepting existing aliases in capture and help text
• reuse backend action-label rendering so custom shortcut labels display consistently across the UI

Validation:

• "$HOME/.local/share/Mouser/venv/bin/python" -m unittest tests.test_backend.BackendDeviceLayoutTests.test_shortcut_capture_accepts_qt_enum_objects tests.test_backend.BackendDeviceLayoutTests.test_shortcut_capture_keeps_ctrl_and_super_distinct_on_macos tests.test_backend.BackendDeviceLayoutTests.test_shortcut_capture_preserves_default_qt_modifier_names_off_macos tests.test_key_simulator.CustomShortcutCaptureTests
• rebuilt the local macOS app with PyInstaller and validated ~/.local/bin/mouser --hid-backend=bogus returns the expected invalid-backend failure