fix(tax): Tax rates now saving correctly + shadcn Select

## Fixed Critical Issues:

### 1. Tax Rates Not Appearing (FIXED ✅)
**Root Cause:** get_tax_rates() was filtering by tax_class, but empty tax_class (standard) was not matching.

**Solution:** Modified get_tax_rates() to treat empty string as standard class:
```php
if ( $tax_class === 'standard' ) {
    // Match both empty string and 'standard'
    WHERE tax_rate_class = '' OR tax_rate_class = 'standard'
}
```

### 2. Select Dropdown Not Using Shadcn (FIXED ✅)
**Problem:** Native select with manual styling was inconsistent.

**Solution:**
- Added selectedTaxClass state
- Used controlled shadcn Select component
- Initialize state when dialog opens/closes
- Pass state value to API instead of form data

## Changes:
- **Backend:** Fixed get_tax_rates() SQL query
- **Frontend:** Converted to controlled Select with state
- **UX:** Tax rates now appear immediately after creation

## Testing:
- ✅ Add tax rate manually
- ✅ Add suggested tax rate
- ✅ Rates appear in list
- ✅ Select dropdown uses shadcn styling

includes/Api/TaxController.php

@@ -171,14 +171,23 @@ class TaxController extends WP_REST_Controller {
 	private function get_tax_rates( $tax_class ) {
 		global $wpdb;
 
-		$rates = $wpdb->get_results(
-			$wpdb->prepare(
+		// For 'standard' class, match both empty string and 'standard'
+		if ( $tax_class === 'standard' ) {
+			$rates = $wpdb->get_results(
 				"SELECT * FROM {$wpdb->prefix}woocommerce_tax_rates 
-				WHERE tax_rate_class = %s 
-				ORDER BY tax_rate_order ASC",
-				$tax_class
-			)
-		);
+				WHERE tax_rate_class = '' OR tax_rate_class = 'standard'
+				ORDER BY tax_rate_order ASC"
+			);
+		} else {
+			$rates = $wpdb->get_results(
+				$wpdb->prepare(
+					"SELECT * FROM {$wpdb->prefix}woocommerce_tax_rates 
+					WHERE tax_rate_class = %s 
+					ORDER BY tax_rate_order ASC",
+					$tax_class
+				)
+			);
+		}
 
 		$formatted_rates = array();