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/routes/Marketing/Coupons/Edit.tsx

@@ -0,0 +1,113 @@
+import React, { useRef, useEffect } from 'react';
+import { useNavigate, useParams } from 'react-router-dom';
+import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import { __ } from '@/lib/i18n';
+import { CouponsApi, type CouponFormData } from '@/lib/api/coupons';
+import { showErrorToast, showSuccessToast, getPageLoadErrorMessage } from '@/lib/errorHandling';
+import { ErrorCard } from '@/components/ErrorCard';
+import { LoadingState } from '@/components/LoadingState';
+import { usePageHeader } from '@/contexts/PageHeaderContext';
+import { Button } from '@/components/ui/button';
+import { useFABConfig } from '@/hooks/useFABConfig';
+import CouponForm from './CouponForm';
+
+export default function CouponEdit() {
+  const { id } = useParams<{ id: string }>();
+  const couponId = Number(id);
+  const navigate = useNavigate();
+  const queryClient = useQueryClient();
+  const formRef = useRef<HTMLFormElement>(null);
+  const { setPageHeader, clearPageHeader } = usePageHeader();
+
+  // Hide FAB on edit page
+  useFABConfig('none');
+
+  // Fetch coupon
+  const { data: coupon, isLoading, isError, error } = useQuery({
+    queryKey: ['coupon', couponId],
+    queryFn: () => CouponsApi.get(couponId),
+    enabled: !!couponId,
+  });
+
+  // Update mutation
+  const updateMutation = useMutation({
+    mutationFn: (data: CouponFormData) => CouponsApi.update(couponId, data),
+    onSuccess: (updatedCoupon) => {
+      queryClient.invalidateQueries({ queryKey: ['coupons'] });
+      queryClient.invalidateQueries({ queryKey: ['coupon', couponId] });
+      showSuccessToast(__('Coupon updated successfully'), `${__('Coupon')} ${updatedCoupon.code} ${__('updated')}`);
+      navigate('/coupons');
+    },
+    onError: (error) => {
+      showErrorToast(error);
+    },
+  });
+
+  // Smart back handler: go back in history if available, otherwise fallback to /coupons
+  const handleBack = () => {
+    if (window.history.state?.idx > 0) {
+      navigate(-1); // Go back in history
+    } else {
+      navigate('/coupons'); // Fallback to coupons index
+    }
+  };
+
+  // Set contextual header
+  useEffect(() => {
+    const actions = (
+      <div className="flex gap-2">
+        <Button size="sm" variant="ghost" onClick={handleBack}>
+          {__('Back')}
+        </Button>
+        <Button
+          size="sm"
+          onClick={() => formRef.current?.requestSubmit()}
+          disabled={updateMutation.isPending || isLoading}
+        >
+          {updateMutation.isPending ? __('Saving...') : __('Save')}
+        </Button>
+      </div>
+    );
+
+    const title = coupon ? `${__('Edit Coupon')}: ${coupon.code}` : __('Edit Coupon');
+    setPageHeader(title, actions);
+    return () => clearPageHeader();
+  }, [coupon, updateMutation.isPending, isLoading, setPageHeader, clearPageHeader, navigate]);
+
+  if (isLoading) {
+    return <LoadingState message={__('Loading coupon...')} />;
+  }
+
+  if (isError) {
+    return (
+      <ErrorCard
+        title={__('Failed to load coupon')}
+        message={getPageLoadErrorMessage(error)}
+        onRetry={() => queryClient.invalidateQueries({ queryKey: ['coupon', couponId] })}
+      />
+    );
+  }
+
+  if (!coupon) {
+    return (
+      <ErrorCard
+        title={__('Coupon not found')}
+        message={__('The requested coupon could not be found')}
+      />
+    );
+  }
+
+  return (
+    <div>
+      <CouponForm
+        mode="edit"
+        initial={coupon}
+        onSubmit={async (data) => {
+          await updateMutation.mutateAsync(data);
+        }}
+        formRef={formRef}
+        hideSubmitButton={true}
+      />
+    </div>
+  );
+}