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/Orders/partials/OrderForm.tsx

@@ -149,13 +149,15 @@ export default function OrderForm({
     if (oneCountryOnly) {
       const only = countries[0]?.code || '';
       if (shipDiff) {
-        if (only && sCountry !== only) setSCountry(only);
+        if (only && shippingData.country !== only) {
+          setShippingData({...shippingData, country: only});
+        }
       } else {
         // keep shipping synced to billing when not different
-        setSCountry(bCountry);
+        setShippingData({...shippingData, country: bCountry});
       }
     }
-  }, [oneCountryOnly, countries, shipDiff, bCountry, sCountry]);
+  }, [oneCountryOnly, countries, shipDiff, bCountry, shippingData.country]);
 
   // Order meta
   const [status, setStatus] = React.useState(initial?.status || 'pending');
@@ -292,7 +294,7 @@ export default function OrderForm({
 
   // Keep shipping country synced to billing when unchecked
   React.useEffect(() => {
-    if (!shipDiff) setSCountry(bCountry);
+    if (!shipDiff) setShippingData({...shippingData, country: bCountry});
   }, [shipDiff, bCountry]);
 
   // Clamp states when country changes
@@ -300,12 +302,13 @@ export default function OrderForm({
     if (bState && !states[bCountry]?.[bState]) setBState('');
   }, [bCountry]);
   React.useEffect(() => {
-    if (sState && !states[sCountry]?.[sState]) setSState('');
-  }, [sCountry]);
+    if (shippingData.state && !states[shippingData.country]?.[shippingData.state]) {
+      setShippingData({...shippingData, state: ''});
+    }
+  }, [shippingData.country]);
 
   const countryOptions = countries.map(c => ({ value: c.code, label: `${c.name} (${c.code})` }));
   const bStateOptions = Object.entries(states[bCountry] || {}).map(([code, name]) => ({ value: code, label: name }));
-  const sStateOptions = Object.entries(states[sCountry] || {}).map(([code, name]) => ({ value: code, label: name }));
 
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault();
@@ -692,13 +695,15 @@ export default function OrderForm({
                         // Autofill shipping if available
                         if (data.shipping && data.shipping.address_1) {
                           setShipDiff(true);
-                          setSFirst(data.shipping.first_name || '');
-                          setSLast(data.shipping.last_name || '');
-                          setSAddr1(data.shipping.address_1 || '');
-                          setSCity(data.shipping.city || '');
-                          setSPost(data.shipping.postcode || '');
-                          setSCountry(data.shipping.country || bCountry);
-                          setSState(data.shipping.state || '');
+                          setShippingData({
+                            first_name: data.shipping.first_name || '',
+                            last_name: data.shipping.last_name || '',
+                            address_1: data.shipping.address_1 || '',
+                            city: data.shipping.city || '',
+                            postcode: data.shipping.postcode || '',
+                            country: data.shipping.country || bCountry,
+                            state: data.shipping.state || '',
+                          });
                         }
                       }