fix: Add region search filter + pre-select on edit + create plan doc

## ✅ Issue #1: TAX_NOTIFICATIONS_PLAN.md Created
- Complete implementation plan for Tax & Notifications
- 80/20 rule: Core features vs Advanced (WooCommerce)
- API endpoints defined
- Implementation phases prioritized

## ✅ Issue #2: Region Search Filter
- Added search input above region list
- Real-time filtering as you type
- Shows "No regions found" when no matches
- Clears search on dialog close/cancel
- Makes finding countries/states MUCH faster!

## ✅ Issue #3: Pre-select Regions on Edit
- Backend now returns raw `locations` array
- Frontend uses `defaultChecked` with location matching
- Existing regions auto-selected when editing zone
- Works correctly for countries, states, and continents

## UX Improvements:
- Search placeholder: "Search regions..."
- Filter is case-insensitive
- Empty state when no results
- Clean state management (clear on close)

Now zone editing is smooth and fast!

includes/Api/ShippingController.php

@@ -229,8 +229,16 @@ class ShippingController extends WP_REST_Controller {
 				// Get zone locations (regions)
 				$locations = $zone->get_zone_locations();
 				$regions = array();
+				$raw_locations = array();
 				
 				foreach ( $locations as $location ) {
+					// Store raw location data for editing
+					$raw_locations[] = array(
+						'code' => $location->code,
+						'type' => $location->type,
+					);
+					
+					// Format for display
 					if ( $location->type === 'country' ) {
 						$countries = WC()->countries->get_countries();
 						$regions[] = $countries[ $location->code ] ?? $location->code;
@@ -280,11 +288,12 @@ class ShippingController extends WP_REST_Controller {
 				}
 				
 				$zones_data[] = array(
-					'id'      => $zone_data['id'],
-					'name'    => $zone->get_zone_name(),
-					'order'   => $zone->get_zone_order(),
-					'regions' => ! empty( $regions ) ? implode( ', ', $regions ) : __( 'No regions', 'woonoow' ),
-					'rates'   => $rates,
+					'id'        => $zone_data['id'],
+					'name'      => $zone->get_zone_name(),
+					'order'     => $zone->get_zone_order(),
+					'regions'   => ! empty( $regions ) ? implode( ', ', $regions ) : __( 'No regions', 'woonoow' ),
+					'locations' => $raw_locations,
+					'rates'     => $rates,
 				);
 			}