Fixes #15569

This PR adds an optional preference to automatically start recording when
Live Broadcasting is enabled.

Summary of changes

• Added a new checkbox in Preferences → Live Broadcasting:
  “Automatically start recording when Live Broadcasting starts.”
• Extended DlgPrefBroadcast to load and save this new setting via ConfigKey.
• Updated BroadcastManager to:
  • Check the new preference when broadcasting switches to enabled
  • Automatically toggle [Recording]toggle_recording if no recording is active
  • Track whether the recording session was auto-started
  • Automatically stop the recording when broadcasting stops (but only if Mixxx
  started it automatically, preserving user intent)

Behavior

• Default: disabled (no behavior change for existing users)
• If enabled:
  • Going live auto-starts recording
  • Stopping the broadcast stops only the auto-started recording
  • Manual recordings are not interrupted

Test plan

• Enable the preference and confirm:
  • Going live starts recording
  • Stopping live stops the auto-started recording
• Confirm that starting a manual recording first does NOT create a second recording.
• Disable the preference and confirm Mixxx behaves exactly as before.