docs: Shipping field hooks + Tax selling locations strategy

## ✅ Issue #1: Shipping Fields - Addon Responsibility

Created SHIPPING_FIELD_HOOKS.md documenting:

**The Right Approach:**
- ❌ NO hardcoding (if country === ID → show subdistrict)
- ✅ YES listen to WooCommerce hooks
- ✅ Addons declare their own field requirements

**How It Works:**
1. Addon adds field via `woocommerce_checkout_fields` filter
2. WooNooW fetches fields via API: `GET /checkout/fields`
3. Frontend renders fields dynamically
4. Validation based on `required` flag

**Benefits:**
- Addon responsibility (not WooNooW)
- No hardcoding assumptions
- Works with ANY addon (Indonesian, UPS, custom)
- Future-proof and extensible

**Example:**
```php
// Indonesian Shipping Addon
add_filter('woocommerce_checkout_fields', function($fields) {
    $fields['shipping']['shipping_subdistrict'] = [
        'required' => true,
        // ...
    ];
    return $fields;
});
```

WooNooW automatically renders it!

## ✅ Issue #2: Tax - Grab Selling Locations

Updated TAX_SETTINGS_DESIGN.md:

**Your Brilliant Idea:**
- Read WooCommerce "Selling location(s)" setting
- Show predefined tax rates for those countries
- No re-selecting!

**Scenarios:**
1. **Specific countries** (ID, MY) → Show both rates
2. **All countries** → Show store country + add button
3. **Continent** (Asia) → Suggest all Asian country rates

**Smart Detection:**
```php
$selling_locations = get_option('woocommerce_allowed_countries');
if ($selling_locations === 'specific') {
    $countries = get_option('woocommerce_specific_allowed_countries');
    // Show predefined rates for these countries
}
```

**Benefits:**
- Zero re-selection (data already in WooCommerce)
- Smart suggestions based on user's actual selling regions
- Scales for single/multi-country/continent
- Combines your idea + my proposal perfectly!

## Next: Implementation Plan Ready

SHIPPING_FIELD_HOOKS.md

@@ -0,0 +1,283 @@
+# Shipping Address Fields - Dynamic via Hooks
+
+## Philosophy: Addon Responsibility, Not Hardcoding
+
+WooNooW should **listen to WooCommerce hooks** to determine which fields are required, not hardcode assumptions about Indonesian vs International shipping.
+
+---
+
+## The Problem with Hardcoding
+
+**Bad Approach (What we almost did):**
+```javascript
+// ❌ DON'T DO THIS
+if (country === 'ID') {
+  showSubdistrict = true; // Hardcoded assumption
+}
+```
+
+**Why it's bad:**
+- Assumes all Indonesian shipping needs subdistrict
+- Breaks if addon changes requirements
+- Not extensible for other countries
+- Violates separation of concerns
+
+---
+
+## The Right Approach: Listen to Hooks
+
+**WooCommerce Core Hooks:**
+
+### 1. `woocommerce_checkout_fields` Filter
+Addons use this to add/modify/remove fields:
+
+```php
+// Example: Indonesian Shipping Addon
+add_filter('woocommerce_checkout_fields', function($fields) {
+    // Add subdistrict field
+    $fields['shipping']['shipping_subdistrict'] = [
+        'label'       => __('Subdistrict'),
+        'required'    => true,
+        'class'       => ['form-row-wide'],
+        'priority'    => 65,
+    ];
+    
+    return $fields;
+});
+```
+
+### 2. `woocommerce_default_address_fields` Filter
+Modifies default address fields:
+
+```php
+add_filter('woocommerce_default_address_fields', function($fields) {
+    // Make postal code required for UPS
+    $fields['postcode']['required'] = true;
+    
+    return $fields;
+});
+```
+
+### 3. Field Validation Hooks
+```php
+add_action('woocommerce_checkout_process', function() {
+    if (empty($_POST['shipping_subdistrict'])) {
+        wc_add_notice(__('Subdistrict is required'), 'error');
+    }
+});
+```
+
+---
+
+## Implementation in WooNooW
+
+### Backend: Expose Checkout Fields via API
+
+**New Endpoint:** `GET /checkout/fields`
+
+```php
+// includes/Api/CheckoutController.php
+
+public function get_checkout_fields(WP_REST_Request $request) {
+    // Get fields with all filters applied
+    $fields = WC()->checkout()->get_checkout_fields();
+    
+    // Format for frontend
+    $formatted = [];
+    
+    foreach ($fields as $fieldset_key => $fieldset) {
+        foreach ($fieldset as $key => $field) {
+            $formatted[] = [
+                'key'         => $key,
+                'fieldset'    => $fieldset_key, // billing, shipping, account, order
+                'type'        => $field['type'] ?? 'text',
+                'label'       => $field['label'] ?? '',
+                'placeholder' => $field['placeholder'] ?? '',
+                'required'    => $field['required'] ?? false,
+                'class'       => $field['class'] ?? [],
+                'priority'    => $field['priority'] ?? 10,
+                'options'     => $field['options'] ?? null, // For select fields
+                'custom'      => $field['custom'] ?? false, // Custom field flag
+            ];
+        }
+    }
+    
+    // Sort by priority
+    usort($formatted, function($a, $b) {
+        return $a['priority'] <=> $b['priority'];
+    });
+    
+    return new WP_REST_Response($formatted, 200);
+}
+```
+
+### Frontend: Dynamic Field Rendering
+
+**Create Order - Address Section:**
+
+```typescript
+// Fetch checkout fields from API
+const { data: checkoutFields = [] } = useQuery({
+  queryKey: ['checkout-fields'],
+  queryFn: () => api.get('/checkout/fields'),
+});
+
+// Filter shipping fields
+const shippingFields = checkoutFields.filter(
+  field => field.fieldset === 'shipping'
+);
+
+// Render dynamically
+{shippingFields.map(field => {
+  // Standard WooCommerce fields
+  if (['first_name', 'last_name', 'address_1', 'address_2', 'city', 'state', 'postcode', 'country'].includes(field.key)) {
+    return <StandardField key={field.key} field={field} />;
+  }
+  
+  // Custom fields (e.g., subdistrict from addon)
+  if (field.custom) {
+    return <CustomField key={field.key} field={field} />;
+  }
+  
+  return null;
+})}
+```
+
+**Field Components:**
+
+```typescript
+function StandardField({ field }) {
+  return (
+    <div className={cn('form-field', field.class)}>
+      <label>
+        {field.label}
+        {field.required && <span className="required">*</span>}
+      </label>
+      <input
+        type={field.type}
+        name={field.key}
+        placeholder={field.placeholder}
+        required={field.required}
+      />
+    </div>
+  );
+}
+
+function CustomField({ field }) {
+  // Handle custom field types (select, textarea, etc.)
+  if (field.type === 'select') {
+    return (
+      <div className={cn('form-field', field.class)}>
+        <label>
+          {field.label}
+          {field.required && <span className="required">*</span>}
+        </label>
+        <select name={field.key} required={field.required}>
+          {field.options?.map(option => (
+            <option key={option.value} value={option.value}>
+              {option.label}
+            </option>
+          ))}
+        </select>
+      </div>
+    );
+  }
+  
+  return <StandardField field={field} />;
+}
+```
+
+---
+
+## How Addons Work
+
+### Example: Indonesian Shipping Addon
+
+**Addon adds subdistrict field:**
+```php
+add_filter('woocommerce_checkout_fields', function($fields) {
+    $fields['shipping']['shipping_subdistrict'] = [
+        'type'        => 'select',
+        'label'       => __('Subdistrict'),
+        'required'    => true,
+        'class'       => ['form-row-wide'],
+        'priority'    => 65,
+        'options'     => get_subdistricts(), // Addon provides this
+        'custom'      => true, // Flag as custom field
+    ];
+    
+    return $fields;
+});
+```
+
+**WooNooW automatically:**
+1. Fetches fields via API
+2. Sees `shipping_subdistrict` with `required: true`
+3. Renders it in Create Order form
+4. Validates it on submit
+
+**No hardcoding needed!**
+
+---
+
+## Benefits
+
+✅ **Addon responsibility** - Addons declare their own requirements
+✅ **No hardcoding** - WooNooW just renders what WooCommerce says
+✅ **Extensible** - Works with ANY addon (Indonesian, UPS, custom)
+✅ **Future-proof** - New addons work automatically
+✅ **Separation of concerns** - Each addon manages its own fields
+
+---
+
+## Edge Cases
+
+### Case 1: Subdistrict for Indonesian Shipping
+- Addon adds `shipping_subdistrict` field
+- WooNooW renders it
+- ✅ Works!
+
+### Case 2: UPS Requires Postal Code
+- UPS addon sets `postcode.required = true`
+- WooNooW renders it as required
+- ✅ Works!
+
+### Case 3: Custom Shipping Needs Extra Field
+- Addon adds `shipping_delivery_notes` field
+- WooNooW renders it
+- ✅ Works!
+
+### Case 4: No Custom Fields
+- Standard WooCommerce fields only
+- WooNooW renders them
+- ✅ Works!
+
+---
+
+## Implementation Plan
+
+1. **Backend:**
+   - Create `GET /checkout/fields` endpoint
+   - Return fields with all filters applied
+   - Include field metadata (type, required, options, etc.)
+
+2. **Frontend:**
+   - Fetch checkout fields on Create Order page
+   - Render fields dynamically based on API response
+   - Handle standard + custom field types
+   - Validate based on `required` flag
+
+3. **Testing:**
+   - Test with no addons (standard fields only)
+   - Test with Indonesian shipping addon (subdistrict)
+   - Test with UPS addon (postal code required)
+   - Test with custom addon (custom fields)
+
+---
+
+## Next Steps
+
+1. Create `CheckoutController.php` with `get_checkout_fields` endpoint
+2. Update Create Order to fetch and render fields dynamically
+3. Test with Indonesian shipping addon
+4. Document for addon developers

TAX_SETTINGS_DESIGN.md

@@ -48,9 +48,42 @@ Tax Settings Page
 
 ---
 
-## Predefined Tax Rates by Country
+## Predefined Tax Rates - Smart Detection
 
-When user's store country is set, we show the standard rate:
+**Source:** WooCommerce General Settings → "Selling location(s)"
+
+### Scenario 1: Sell to Specific Countries
+If user selected specific countries (e.g., Indonesia, Malaysia):
+```
+Predefined Tax Rates
+├── 🇮🇩 Indonesia: 11% (PPN)
+└── 🇲🇾 Malaysia: 6% (SST)
+```
+
+### Scenario 2: Sell to All Countries
+If user selected "Sell to all countries":
+```
+Predefined Tax Rates
+└── Based on store country:
+    🇮🇩 Indonesia: 11% (PPN)
+
+Additional Tax Rates
+└── [+ Add Tax Rate] → Shows all countries
+```
+
+### Scenario 3: Sell to Specific Continents
+If user selected "Asia":
+```
+Suggested Tax Rates (Asia)
+├── 🇮🇩 Indonesia: 11%
+├── 🇲🇾 Malaysia: 6%
+├── 🇸🇬 Singapore: 9%
+├── 🇹🇭 Thailand: 7%
+├── 🇵🇭 Philippines: 12%
+└── 🇻🇳 Vietnam: 10%
+```
+
+### Standard Tax Rates by Country
 
 | Country | Standard Rate | Note |
 |---------|---------------|------|
@@ -124,7 +157,23 @@ Add Tax Rate
 - `POST /settings/tax/rates` - Create tax rate
 - `PUT /settings/tax/rates/{id}` - Update tax rate
 - `DELETE /settings/tax/rates/{id}` - Delete tax rate
-- `GET /settings/tax/predefined` - Get predefined rates by country
+- `GET /settings/tax/suggested` - Get suggested rates based on selling locations
+
+### Get Selling Locations:
+```php
+// Get WooCommerce selling locations setting
+$selling_locations = get_option('woocommerce_allowed_countries');
+// Options: 'all', 'all_except', 'specific'
+
+if ($selling_locations === 'specific') {
+    $countries = get_option('woocommerce_specific_allowed_countries');
+    // Returns array: ['ID', 'MY', 'SG']
+}
+
+// Get store base country
+$store_country = get_option('woocommerce_default_country');
+// Returns: 'ID:JB' (country:state) or 'ID'
+```
 
 ### Predefined Rates Data:
 ```json