Suspend arrow keys while ruler settings are open

[pascalpp]

Fixes #272.

Summary

• Replaced the stale preferencesWindowOpen boolean with owner-tracked ruler interaction suspension.
• Suspended only the ruler attached to the per-ruler Settings panel, and moved that suspension when the reusable panel switches rulers.
• Let arrow-key events pass through while ruler interaction is suspended or the ruler window is not key, so settings controls can handle navigation.

Root Cause

The previous state model only represented the global Preferences window. After multiple rulers and the per-ruler Settings panel were introduced, the key listener could still consume arrow keys for ruler nudging while the Settings panel was open.

Validation

• xcodebuild -project "Free Ruler.xcodeproj" -scheme "Free Ruler" test -only-testing:FreeRulerTests
• git diff --check