Own and dispose app-level stream subscriptions

[robekl]

Summary

Owns and disposes long-lived AppStateProvider stream subscriptions to prevent leaked listeners and duplicate callbacks across provider lifecycle events.

Problem

AppStateProvider created listeners for:

• Bluetooth connectionStream
• GPS statusStream
• GPS positionStream

but did not keep/cancel those subscriptions explicitly in dispose().
That can cause lingering listeners and repeated state updates after re-init/hot-reload/provider recreation.

Changes

• Added subscription fields:
  • _connectionSubscription
  • _gpsStatusSubscription
  • _gpsPositionSubscription
• In _initialize():
  • cancel existing subscription before re-subscribing for each stream
  • store new subscription handles
• In dispose():
  • explicitly cancel all three new subscriptions

Why this works

All app-level stream listeners are now lifecycle-owned by the provider instance and deterministically cancelled at teardown.