feat: add double-tap to trigger recording

[DehnP]

Rationale

I like using all my keys for other things, but I don't like being a crab to press combinations; double-tap works nicely for me. Maybe it will for others :)

Summary

• Add a "Double Tap to Trigger" setting that requires two quick presses of the modifier-only hotkey (within the system double-click interval) to start recording, reducing accidental activation
• Once recording is active, a single press still stops it, so there's no need to double-tap again to stop
• Persist the preference via AppPreferences.doublePressToTrigger and expose a toggle in Settings

Test plan

• Enable "Double Tap to Trigger" in Settings and confirm a single press no longer starts recording
• Confirm two quick presses (within the OS double-click interval) starts recording
• Confirm a single press while recording still stops it
• Confirm the setting persists across app restarts
• Confirm existing single-tap behavior is unchanged when the setting is off