fix: stabilize recording teardown and escape hotkey behavior

[meck93]

Summary

• move camera preview session start/stop work off the main thread and detach the preview layer during teardown
• keep the global Escape cancel hotkey limited to source picking/countdown instead of active recording
• clean up related Swift 6 warning paths in app delegate and hotkey setup

Verification

• mise run release-build
• mise run lint
• mise run build
• mise run test