Moves tray/gadget sensor selection (the per-sensor 'tray'/'gadget' settings,
their key convention, and the GadgetSensorsChanged/TraySensorsChanged events)
into a focused, testable collaborator. The view model keeps thin delegating
methods and forwards the events, so its public surface (used by MainWindow) is
unchanged. Behavior is unchanged.
Adds 3 tests (persistence round-trip, event raising, tray filtering).
196 tests pass (was 193).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
