feat(i18n): Comprehensive internationalization of frontend components and pages

Translate all hardcoded English strings to use i18n translation keys:

Components:
- TransactionDetailModal: payment details, refunds, technical info
- ConnectOnboarding/ConnectOnboardingEmbed: Stripe Connect setup
- StripeApiKeysForm: API key management
- DomainPurchase: domain registration flow
- Sidebar: navigation labels
- Schedule/Sidebar, PendingSidebar: scheduler UI
- MasqueradeBanner: masquerade status
- Dashboard widgets: metrics, capacity, customers, tickets
- Marketing: PricingTable, PluginShowcase, BenefitsSection
- ConfirmationModal, ServiceList: common UI

Pages:
- Staff: invitation flow, role management
- Customers: form placeholders
- Payments: transactions, payouts, billing
- BookingSettings: URL and redirect configuration
- TrialExpired: upgrade prompts and features
- PlatformSettings, PlatformBusinesses: admin UI
- HelpApiDocs: API documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/ConfirmationModal.tsx

@@ -1,5 +1,6 @@
 import React from 'react';
 import { X, AlertTriangle, CheckCircle, Info, AlertCircle } from 'lucide-react';
+import { useTranslation } from 'react-i18next';
 
 type ModalVariant = 'info' | 'warning' | 'danger' | 'success';
 
@@ -48,11 +49,13 @@ const ConfirmationModal: React.FC<ConfirmationModalProps> = ({
   onConfirm,
   title,
   message,
-  confirmText = 'Confirm',
-  cancelText = 'Cancel',
+  confirmText,
+  cancelText,
   variant = 'info',
   isLoading = false,
 }) => {
+  const { t } = useTranslation();
+
   if (!isOpen) return null;
 
   const config = variantConfig[variant];
@@ -95,7 +98,7 @@ const ConfirmationModal: React.FC<ConfirmationModalProps> = ({
             disabled={isLoading}
             className="px-4 py-2 text-gray-700 dark:text-gray-300 border border-gray-300 dark:border-gray-600 rounded-lg hover:bg-gray-50 dark:hover:bg-gray-700 transition-colors disabled:opacity-50"
           >
-            {cancelText}
+            {cancelText || t('common.cancel')}
           </button>
           <button
             onClick={handleConfirm}
@@ -120,7 +123,7 @@ const ConfirmationModal: React.FC<ConfirmationModalProps> = ({
                 />
               </svg>
             )}
-            {confirmText}
+            {confirmText || t('common.confirm')}
           </button>
         </div>
       </div>