Add configurable QoS for the managed Syncthing process

[robios]

Summary

• add Normal, Utility, and Background priority choices to Preferences
• persist the selected quality of service and apply it when launching Syncthing
• restart the managed Syncthing process when the setting changes
• preserve Background as the fallback for existing installations
• document the new preference

Why

The wrapper currently launches Syncthing with background QoS unconditionally.
On systems with large folders, macOS background scheduling can make full scans
substantially slower. Normal and Utility allow users to trade energy impact for
scan performance without changing Syncthing's own configuration.

Closes #293

Testing

• make debug
• launched the resulting app on macOS 15.6 (Apple Silicon)
• switched between Normal, Utility, and Background
• verified that changing the setting restarts Syncthing
• verified that the Syncthing health endpoint returns OK
• used Normal and Utility with a folder containing about 393,000 local files;
  both avoid the severe slowdown observed with Background

Compatibility

If the preference has never been set, or contains an unknown value, the app
continues to use Background. This preserves the behavior of existing
installations.

Draft status

This is ready for design feedback. Additional controlled scan timings and
longer-running Utility usage can be added before marking the PR ready for
review.