Make recognition hotkey configurable

[shhtheonlyperson]

Summary

• add a shared RecognitionHotkey model for configurable modifier-only recognition shortcuts
• replace the static Right Option settings row with an explicit dropdown menu
• support configurable Option, Control, Command, and Shift recognition shortcuts; Fn/Globe is intentionally not included because it does not behave consistently through the current event path
• route hotkey handling, status bar hints, setup messaging, and docs through the configured shortcut
• allow build-app.sh to reuse a stable local TypelessMLX Local Code Signing identity so rebuilds do not fall back to ad-hoc signing

Verification

• swift build
• git diff --check
• swift run TypelessMLXAudioTapFormatTests
• swift run TypelessMLXAudioInputAvailabilityTests
• ./build-app.sh --install picks TypelessMLX Local Code Signing and installs /Applications/TypelessMLX.app with stable designated requirement identifier "com.typelessmlx.app" and certificate root = H"33a86247fe8737fc69ccac04f9e79917fd25a46a"

Notes

• The local signing certificate was created in the user login keychain on this machine because no Apple Development identity was available.
• This PR was prepared from a clean worktree so it does not include unrelated local checkout changes.