fix: Prevent cascade clearing of VIN decoded form values (refs #9)

VIN decode was setting year/make/model/trim values, but the cascading
dropdown useEffects would immediately clear dependent fields because
they detected a value change. Added isVinDecoding ref flag (mirroring
the existing isInitializing pattern for edit mode) to skip cascade
clearing during VIN decode and properly load dropdown options.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/vehicles/components/VehicleForm.tsx

@@ -95,6 +95,7 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
   const [loadingDropdowns, setLoadingDropdowns] = useState(false);
   const hasInitialized = useRef(false);
   const isInitializing = useRef(false);
+  const isVinDecoding = useRef(false);
   // Track previous values for cascade change detection (replaces useState)
   const prevYear = useRef<number | undefined>(undefined);
   const prevMake = useRef<string>('');
@@ -224,8 +225,8 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
 
   // Load makes when year changes
   useEffect(() => {
-    // Skip during initialization
-    if (isInitializing.current) return;
+    // Skip during initialization or VIN decoding
+    if (isInitializing.current || isVinDecoding.current) return;
 
     if (watchedYear && watchedYear !== prevYear.current) {
       const loadMakes = async () => {
@@ -262,8 +263,8 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
 
   // Load models when make changes
   useEffect(() => {
-    // Skip during initialization
-    if (isInitializing.current) return;
+    // Skip during initialization or VIN decoding
+    if (isInitializing.current || isVinDecoding.current) return;
 
     if (watchedMake && watchedYear && watchedMake !== prevMake.current) {
       const loadModels = async () => {
@@ -297,8 +298,8 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
 
   // Load trims when model changes
   useEffect(() => {
-    // Skip during initialization
-    if (isInitializing.current) return;
+    // Skip during initialization or VIN decoding
+    if (isInitializing.current || isVinDecoding.current) return;
 
     if (watchedModel && watchedYear && watchedMake && watchedModel !== prevModel.current) {
       const loadTrims = async () => {
@@ -329,8 +330,8 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
 
   // Load engines and transmissions when trim changes
   useEffect(() => {
-    // Skip during initialization
-    if (isInitializing.current) return;
+    // Skip during initialization or VIN decoding
+    if (isInitializing.current || isVinDecoding.current) return;
 
     if (watchedTrim && watchedYear && watchedMake && watchedModel && watchedTrim !== prevTrim.current) {
       const loadEnginesAndTransmissions = async () => {
@@ -443,7 +444,17 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
     try {
       const decoded = await vehiclesApi.decodeVin(vin);
 
-      // Only populate empty fields (preserve existing user input)
+      // Prevent cascade useEffects from clearing values during VIN decode
+      isVinDecoding.current = true;
+      setLoadingDropdowns(true);
+
+      // Track which values we're setting (only populate empty fields)
+      const yearToSet = !watchedYear && decoded.year.value ? decoded.year.value : watchedYear;
+      const makeToSet = !watchedMake && decoded.make.value ? decoded.make.value : watchedMake;
+      const modelToSet = !watchedModel && decoded.model.value ? decoded.model.value : watchedModel;
+      const trimToSet = !watchedTrim && decoded.trimLevel.value ? decoded.trimLevel.value : watchedTrim;
+
+      // Set form values
       if (!watchedYear && decoded.year.value) {
         setValue('year', decoded.year.value);
       }
@@ -463,16 +474,38 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
         setValue('transmission', decoded.transmission.value);
       }
 
-      // Body type, drive type, fuel type - check if fields are empty and we have values
-      const currentDriveType = watch('driveType');
-      const currentFuelType = watch('fuelType');
+      // Load dropdown options hierarchically (like edit mode initialization)
+      // This ensures dropdowns have the right options for the decoded values
+      if (yearToSet) {
+        prevYear.current = yearToSet;
+        const makesData = await vehiclesApi.getMakes(yearToSet);
+        setMakes(makesData);
 
-      if (!currentDriveType && decoded.driveType.nhtsaValue) {
-        // For now just show hint - user can select from dropdown
-      }
-      if (!currentFuelType && decoded.fuelType.nhtsaValue) {
-        // For now just show hint - user can select from dropdown
+        if (makeToSet) {
+          prevMake.current = makeToSet;
+          const modelsData = await vehiclesApi.getModels(yearToSet, makeToSet);
+          setModels(modelsData);
+
+          if (modelToSet) {
+            prevModel.current = modelToSet;
+            const trimsData = await vehiclesApi.getTrims(yearToSet, makeToSet, modelToSet);
+            setTrims(trimsData);
+
+            if (trimToSet) {
+              prevTrim.current = trimToSet;
+              const [enginesData, transmissionsData] = await Promise.all([
+                vehiclesApi.getEngines(yearToSet, makeToSet, modelToSet, trimToSet),
+                vehiclesApi.getTransmissions(yearToSet, makeToSet, modelToSet, trimToSet)
+              ]);
+              setEngines(enginesData);
+              setTransmissions(transmissionsData);
+            }
+          }
+        }
       }
+
+      setLoadingDropdowns(false);
+      isVinDecoding.current = false;
     } catch (error: any) {
       console.error('VIN decode failed:', error);
       if (error.response?.data?.error === 'TIER_REQUIRED') {
@@ -484,6 +517,8 @@ export const VehicleForm: React.FC<VehicleFormProps> = ({
       }
     } finally {
       setIsDecoding(false);
+      setLoadingDropdowns(false);
+      isVinDecoding.current = false;
     }
   };