fix: Resolve Tax and OrderForm errors

## Error 1: Tax Settings - Empty SelectItem value ✅
**Issue:** Radix UI Select does not allow empty string as SelectItem value
**Error:** "A <Select.Item /> must have a value prop that is not an empty string"

**Solution:**
- Use 'standard' instead of empty string for UI
- Convert 'standard' → '' when submitting to API
- Initialize selectedTaxClass to 'standard'
- Update all dialog handlers to use 'standard'

## Error 2: OrderForm - Undefined shipping variables ✅
**Issue:** Removed individual shipping state variables (sFirst, sLast, sCountry, etc.) but forgot to update all references
**Error:** "Cannot find name 'sCountry'"

**Solution:**
Fixed all remaining references:
1. **useEffect for country sync:** `setSCountry(bCountry)` → `setShippingData({...shippingData, country: bCountry})`
2. **useEffect for state validation:** `sState && !states[sCountry]` → `shippingData.state && !states[shippingData.country]`
3. **Customer autofill:** Individual setters → `setShippingData({ first_name, last_name, ... })`
4. **Removed sStateOptions:** No longer needed with dynamic fields

## Testing:
- ✅ Tax settings page loads without errors
- ✅ Add/Edit tax rate dialog works
- ✅ OrderForm loads without errors
- ✅ Shipping fields render dynamically
- ✅ Customer autofill works with new state structure

admin-spa/src/routes/Settings/Tax.tsx

@@ -24,7 +24,7 @@ export default function TaxSettings() {
   const [editingRate, setEditingRate] = useState<any | null>(null);
   const [deletingRate, setDeletingRate] = useState<any | null>(null);
   const [dismissedSuggestions, setDismissedSuggestions] = useState<string[]>([]);
-  const [selectedTaxClass, setSelectedTaxClass] = useState<string>('');
+  const [selectedTaxClass, setSelectedTaxClass] = useState<string>('standard');
 
   // Fetch tax settings
   const { data: settings, isLoading } = useQuery({
@@ -142,7 +142,7 @@ export default function TaxSettings() {
       state: formData.get('state') as string || '',
       rate: parseFloat(formData.get('rate') as string),
       name: formData.get('name') as string,
-      tax_class: selectedTaxClass || '',
+      tax_class: selectedTaxClass === 'standard' ? '' : selectedTaxClass,
       priority: 1,
       compound: 0,
       shipping: 1,
@@ -222,7 +222,7 @@ export default function TaxSettings() {
               <Button
                 size="sm"
                 onClick={() => {
-                  setSelectedTaxClass('');
+                  setSelectedTaxClass('standard');
                   setShowAddRate(true);
                 }}
               >
@@ -396,10 +396,10 @@ export default function TaxSettings() {
         if (!open) {
           setShowAddRate(false);
           setEditingRate(null);
-          setSelectedTaxClass('');
+          setSelectedTaxClass('standard');
         } else {
-          // Initialize tax class when opening
-          setSelectedTaxClass(editingRate?.tax_class || '');
+          // Initialize tax class when opening (convert empty to 'standard')
+          setSelectedTaxClass(editingRate?.tax_class || 'standard');
         }
       }}>
         <DialogContent>
@@ -464,12 +464,12 @@ export default function TaxSettings() {
 
               <div>
                 <Label>{__('Tax Class (Optional)')}</Label>
-                <Select value={selectedTaxClass} onValueChange={setSelectedTaxClass}>
+                <Select value={selectedTaxClass || 'standard'} onValueChange={setSelectedTaxClass}>
                   <SelectTrigger>
                     <SelectValue placeholder={__('Standard')} />
                   </SelectTrigger>
                   <SelectContent>
-                    <SelectItem value="">{__('Standard')}</SelectItem>
+                    <SelectItem value="standard">{__('Standard')}</SelectItem>
                     <SelectItem value="reduced-rate">{__('Reduced Rate')}</SelectItem>
                     <SelectItem value="zero-rate">{__('Zero Rate')}</SelectItem>
                   </SelectContent>