fix: Connect measurement unit preference to backend storage

Fixed measurement unit not persisting across page refreshes:

- Settings page now includes measurementUnit in the preferences object when saving
- MeasurementUnitSelector now accepts value/onChange props for controlled usage
- Settings state properly loads and saves measurementUnit from user preferences
- UnitInput component will now correctly read imperial/metric from user.preferences.measurementUnit

Previously, measurementUnit was only saved to localStorage but not synced to backend,
causing UnitInput to always default to metric since it reads from user.preferences.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

maternal-web/components/forms/UnitInput.tsx

@@ -44,6 +44,9 @@ export function UnitInput({ type, value, onChange, ...textFieldProps }: UnitInpu
   const measurementSystem: MeasurementSystem =
     (user?.preferences?.measurementUnit as MeasurementSystem) || 'metric';
 
+  // Debug logging
+  console.log('[UnitInput] Measurement system:', measurementSystem, 'User preferences:', user?.preferences);
+
   // Get the display unit symbol
   const unitSymbol = getUnitSymbol(type, measurementSystem);