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/MasqueradeBanner.tsx

@@ -1,5 +1,6 @@
 
 import React from 'react';
+import { useTranslation } from 'react-i18next';
 import { Eye, XCircle } from 'lucide-react';
 import { User } from '../types';
 
@@ -11,8 +12,9 @@ interface MasqueradeBannerProps {
 }
 
 const MasqueradeBanner: React.FC<MasqueradeBannerProps> = ({ effectiveUser, originalUser, previousUser, onStop }) => {
-  
-  const buttonText = previousUser ? `Return to ${previousUser.name}` : 'Stop Masquerading';
+  const { t } = useTranslation();
+
+  const buttonText = previousUser ? t('platform.masquerade.returnTo', { name: previousUser.name }) : t('platform.masquerade.stopMasquerading');
 
   return (
     <div className="bg-orange-600 text-white px-4 py-2 shadow-md flex items-center justify-between z-50 relative">
@@ -21,9 +23,9 @@ const MasqueradeBanner: React.FC<MasqueradeBannerProps> = ({ effectiveUser, orig
           <Eye size={18} />
         </div>
         <span className="text-sm font-medium">
-          Masquerading as <strong>{effectiveUser.name}</strong> ({effectiveUser.role}) 
-          <span className="opacity-75 mx-2 text-xs">|</span> 
-          Logged in as {originalUser.name}
+          {t('platform.masquerade.masqueradingAs')} <strong>{effectiveUser.name}</strong> ({effectiveUser.role})
+          <span className="opacity-75 mx-2 text-xs">|</span>
+          {t('platform.masquerade.loggedInAs', { name: originalUser.name })}
         </span>
       </div>
       <button