Fix paste target loss from duplicate hotkey callbacks

[uniplanck]

Summary

• stop redundant global fallback listeners when the Carbon hotkey is registered
• debounce duplicate main shortcut callbacks and preserve the original input target if the panel is already visible
• restore only a settable AX focused element before sending paste
• add regression coverage for duplicate callbacks

Verification

• HotKeyServiceTests: 9/9 passed
• full suite: 72/72 passed
• Universal Release build succeeded for arm64 and x86_64
• installed app passed codesign verification
• TextEdit E2E: opened Board-Man from an active text field, selected an item, and content was inserted into the same field without the previous beep-only failure