Add unit settings, wind particles toggle, and recenter-to-top

- New UnitPrefs: persisted C/F, ft/m, hPa/inHg, kt/mph/kph — defaults to °F
- InstrumentHandler now accepts raw values (knots, metres, hPa, °C) and formats
  via UnitPrefs on every update
- LayerPickerSheet expanded with wind particles toggle and unit chip selectors;
  wrapped in ScrollView to handle the additional height
- MapLayerManager tracks particlesEnabled (persisted); setParticlesEnabled() added
- MainActivity: caches last raw values so refreshUnits() can reformat everything
  instantly when the user changes a unit without waiting for new sensor data
- Recenter button moved to top (below HUD strip) so it's never obscured by the
  bottom sheet or bottom nav
- Unit label TextViews given IDs in HUD and instrument sheet for live updates

https://claude.ai/code/session_01HXPjBsogsJVRwCiekDGkJX

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