feat: Newsletter system improvements and validation framework

- Fix: Marketing events now display in Staff notifications tab
- Reorganize: Move Coupons to Marketing/Coupons for better organization
- Add: Comprehensive email/phone validation with extensible filter hooks
  - Email validation with regex pattern (xxxx@xxxx.xx)
  - Phone validation with WhatsApp verification support
  - Filter hooks for external API integration (QuickEmailVerification, etc.)
- Fix: Newsletter template routes now use centralized notification email builder
- Add: Validation.php class for reusable validation logic
- Add: VALIDATION_HOOKS.md documentation with integration examples
- Add: NEWSLETTER_CAMPAIGN_PLAN.md architecture for future campaign system
- Fix: API delete method call in Newsletter.tsx (delete -> del)
- Remove: Duplicate EmailTemplates.tsx (using notification system instead)
- Update: Newsletter controller to use centralized Validation class

Breaking changes:
- Coupons routes moved from /routes/Coupons to /routes/Marketing/Coupons
- Legacy /coupons routes maintained for backward compatibility

admin-spa/src/App.tsx

@@ -18,9 +18,9 @@ import ProductEdit from '@/routes/Products/Edit';
 import ProductCategories from '@/routes/Products/Categories';
 import ProductTags from '@/routes/Products/Tags';
 import ProductAttributes from '@/routes/Products/Attributes';
-import CouponsIndex from '@/routes/Coupons';
-import CouponNew from '@/routes/Coupons/New';
-import CouponEdit from '@/routes/Coupons/Edit';
+import CouponsIndex from '@/routes/Marketing/Coupons';
+import CouponNew from '@/routes/Marketing/Coupons/New';
+import CouponEdit from '@/routes/Marketing/Coupons/Edit';
 import CustomersIndex from '@/routes/Customers';
 import CustomerNew from '@/routes/Customers/New';
 import CustomerEdit from '@/routes/Customers/Edit';
@@ -250,7 +250,6 @@ import AppearanceThankYou from '@/routes/Appearance/ThankYou';
 import AppearanceAccount from '@/routes/Appearance/Account';
 import MarketingIndex from '@/routes/Marketing';
 import NewsletterSubscribers from '@/routes/Marketing/Newsletter';
-import EmailTemplates from '@/routes/Marketing/EmailTemplates';
 import MorePage from '@/routes/More';
 
 // Addon Route Component - Dynamically loads addon components
@@ -515,10 +514,13 @@ function AppRoutes() {
       <Route path="/orders/:id" element={<OrderDetail />} />
       <Route path="/orders/:id/edit" element={<OrderEdit />} />
 
-      {/* Coupons */}
+      {/* Coupons (under Marketing) */}
       <Route path="/coupons" element={<CouponsIndex />} />
       <Route path="/coupons/new" element={<CouponNew />} />
       <Route path="/coupons/:id/edit" element={<CouponEdit />} />
+      <Route path="/marketing/coupons" element={<CouponsIndex />} />
+      <Route path="/marketing/coupons/new" element={<CouponNew />} />
+      <Route path="/marketing/coupons/:id/edit" element={<CouponEdit />} />
 
       {/* Customers */}
       <Route path="/customers" element={<CustomersIndex />} />
@@ -565,7 +567,6 @@ function AppRoutes() {
       {/* Marketing */}
       <Route path="/marketing" element={<MarketingIndex />} />
       <Route path="/marketing/newsletter" element={<NewsletterSubscribers />} />
-      <Route path="/marketing/newsletter/template/:template" element={<EmailTemplates />} />
 
       {/* Dynamic Addon Routes */}
       {addonRoutes.map((route: any) => (