Fix backend module imports for installed package (#22)

[sjmf]

Summary

• Fix ModuleNotFoundError: No module named 'backend' when running as an installed package (fixes Backend module relative import broken #22)
• Extract dynamic imports in keyboard.py into a _load_implementation() helper with a data-driven _MODE_IMPLEMENTATIONS dict, replacing repetitive try/except blocks
• Read CFBundleVersion from pyproject.toml at PyInstaller build time instead of hardcoding (was stale at 1.5.1)
• Bump version to 1.5.4

Test plan

• All 12 tests/backend/test_keyboard.py and tests/backend/test_mouse.py tests pass
• pip install into a clean venv and run kvm-control --mode tty <port> to verify the import fix end-to-end
• pyinstaller kvm-gui.spec builds and the .app bundle reports the correct version

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.98%. Comparing base (a94660a) to head (958d30c).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   78.18%   77.98%   -0.20%     
==========================================
  Files          19       19              
  Lines        1733     1731       -2     
  Branches      213      210       -3     
==========================================
- Hits         1355     1350       -5     
- Misses        378      381       +3     

Flag	Coverage Δ
unittests	77.98% <100.00%> (-0.20%)	⬇️

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