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!

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

@@ -42,6 +42,7 @@ export default function ShippingPage() {
   const [showAddZone, setShowAddZone] = useState(false);
   const [editingZone, setEditingZone] = useState<any | null>(null);
   const [deletingZone, setDeletingZone] = useState<any | null>(null);
+  const [regionSearch, setRegionSearch] = useState('');
   const isDesktop = useMediaQuery("(min-width: 768px)");
   
   // Fetch shipping zones from WooCommerce
@@ -877,6 +878,7 @@ export default function ShippingPage() {
         if (!open) {
           setShowAddZone(false);
           setEditingZone(null);
+          setRegionSearch('');
         }
       }}>
         <DialogContent className="max-w-2xl max-h-[90vh] overflow-y-auto">
@@ -927,6 +929,16 @@ export default function ShippingPage() {
               <p className="text-xs text-muted-foreground mb-3">
                 {__('Select countries, states, or continents for this zone')}
               </p>
+              
+              {/* Search Filter */}
+              <input
+                type="text"
+                placeholder={__('Search regions...')}
+                value={regionSearch}
+                onChange={(e) => setRegionSearch(e.target.value)}
+                className="w-full px-3 py-2 border rounded-md mb-2"
+              />
+              
               <div className="border rounded-md max-h-[300px] overflow-y-auto">
                 {availableLocations.length === 0 ? (
                   <div className="p-4 text-center text-sm text-muted-foreground">
@@ -934,20 +946,32 @@ export default function ShippingPage() {
                   </div>
                 ) : (
                   <div className="divide-y">
-                    {availableLocations.map((location: any) => (
-                      <label
-                        key={location.code}
-                        className="flex items-center gap-3 p-3 hover:bg-accent cursor-pointer"
-                      >
-                        <input
-                          type="checkbox"
-                          name="regions"
-                          value={location.code}
-                          className="rounded"
-                        />
-                        <span className="text-sm">{location.label}</span>
-                      </label>
-                    ))}
+                    {availableLocations
+                      .filter((location: any) => 
+                        location.label.toLowerCase().includes(regionSearch.toLowerCase())
+                      )
+                      .map((location: any) => (
+                        <label
+                          key={location.code}
+                          className="flex items-center gap-3 p-3 hover:bg-accent cursor-pointer"
+                        >
+                          <input
+                            type="checkbox"
+                            name="regions"
+                            value={location.code}
+                            defaultChecked={editingZone?.locations?.some((l: any) => l.code === location.code)}
+                            className="rounded"
+                          />
+                          <span className="text-sm">{location.label}</span>
+                        </label>
+                      ))}
+                    {availableLocations.filter((location: any) => 
+                      location.label.toLowerCase().includes(regionSearch.toLowerCase())
+                    ).length === 0 && (
+                      <div className="p-4 text-center text-sm text-muted-foreground">
+                        {__('No regions found')}
+                      </div>
+                    )}
                   </div>
                 )}
               </div>
@@ -960,6 +984,7 @@ export default function ShippingPage() {
                 onClick={() => {
                   setShowAddZone(false);
                   setEditingZone(null);
+                  setRegionSearch('');
                 }}
               >
                 {__('Cancel')}