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/settings/MeasurementUnitSelector.tsx

@@ -17,6 +17,8 @@ interface MeasurementUnitSelectorProps {
   variant?: 'standard' | 'outlined' | 'filled';
   fullWidth?: boolean;
   showDescription?: boolean;
+  value?: MeasurementSystem;
+  onChange?: (value: MeasurementSystem) => void;
 }
 
 /**
@@ -26,19 +28,25 @@ export function MeasurementUnitSelector({
   variant = 'outlined',
   fullWidth = true,
   showDescription = true,
+  value,
+  onChange,
 }: MeasurementUnitSelectorProps) {
   const { measurementSystem, setMeasurementSystem } = useLocale();
   const { t } = useTranslation('settings');
-  const [selectedSystem, setSelectedSystem] = useState<MeasurementSystem>(measurementSystem);
 
-  useEffect(() => {
-    setSelectedSystem(measurementSystem);
-  }, [measurementSystem]);
+  // Use controlled value if provided, otherwise use hook value
+  const selectedSystem = value !== undefined ? value : measurementSystem;
 
   const handleSystemChange = (event: SelectChangeEvent) => {
     const newSystem = event.target.value as MeasurementSystem;
-    setSelectedSystem(newSystem);
-    setMeasurementSystem(newSystem);
+
+    // If onChange is provided (controlled), use it
+    if (onChange) {
+      onChange(newSystem);
+    } else {
+      // Otherwise use the hook (uncontrolled)
+      setMeasurementSystem(newSystem);
+    }
   };
 
   return (