Fix function-key transcription shortcuts

[basnijholt]

Summary

• Route transcription hotkeys through a function-aware event tap so Fn, Fn+Space, and plain Space remain distinct.
• Delay bare-Fn hold-to-transcribe startup so the Fn+Space toggle chord can be recognized first.
• Preserve custom hold-to-transcribe shortcuts and document the shortcut handling path.

Test Plan

• uv run pytest tests/test_macos_app.py -q
• swift build --package-path macos/AgentCLI
• swift test --package-path macos/AgentCLI