fix(tax): Initialize selectedTaxClass when opening Add Tax Rate dialog

Fixed blank screen when clicking "Add Tax Rate" button by initializing selectedTaxClass state to empty string before opening dialog.

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

@@ -221,7 +221,10 @@ export default function TaxSettings() {
             action={
               <Button
                 size="sm"
-                onClick={() => setShowAddRate(true)}
+                onClick={() => {
+                  setSelectedTaxClass('');
+                  setShowAddRate(true);
+                }}
               >
                 <Plus className="h-4 w-4 mr-2" />
                 {__('Add Tax Rate')}