Clear omitted shortcuts when importing config

[arhxam]

Summary

• treat an imported shortcut map as the complete desired shortcut state
• remove existing active and Todo-mode shortcuts when their keys are omitted
• clear invalid negative-key-code entries instead of retaining stale bindings
• preserve canonical-name precedence and legacy alias fallback

Fixes #1346.

This also prevents an imported shortcut from silently conflicting with an old binding that was intentionally absent from the exported configuration.

Verification

• ConfigImportTests: 5/5 passing (omitted active, omitted Todo, valid active, alias fallback, invalid active)
• full RectangleTests: 210 tests run; the same 22 pre-existing assertions fail as on main, confined to ActiveSideSplitRatiosCooperativeTests, CooperativeCornerResizeTests, and HalfSplitCornerCalculationTests
• git diff --check origin/main...HEAD

AI assistance

AI tooling assisted with investigation, implementation, and test generation. The final diff passed independent specification and code-quality reviews and was verified locally.