fix: preserve cross-platform input behavior

[johneybi]

Summary

Bring the current cross-platform input fixes onto the latest KeyStitch master in one focused commit.

Changes

• Handle headless client stdin without terminal assertions
• Improve macOS IME layout fallback and extended function-key mapping
• Keep remap configuration explicitly opt-in per target screen
• Reduce Windows cursor transition noise while preserving current high-rate protection
• Add switch delay and double-tap settings to local test configurations

Verification

• Shell and diff checks passed
• Internal live-test baseline was intentionally kept out of the public change
• CI will validate Linux and macOS builds/tests