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/Sidebar.tsx

@@ -59,7 +59,7 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
       <button
         onClick={toggleCollapse}
         className={`flex items-center gap-3 w-full text-left px-6 py-6 ${isCollapsed ? 'justify-center' : ''} hover:bg-white/5 transition-colors focus:outline-none`}
-        aria-label={isCollapsed ? "Expand sidebar" : "Collapse sidebar"}
+        aria-label={isCollapsed ? t('nav.expandSidebar') : t('nav.collapseSidebar')}
       >
         {business.logoDisplayMode === 'logo-only' && business.logoUrl ? (
           <div className="flex items-center justify-center w-full">
@@ -234,7 +234,7 @@ const Sidebar: React.FC<SidebarProps> = ({ business, user, isCollapsed, toggleCo
         >
           <SmoothScheduleLogo className="w-5 h-5 text-white" />
           {!isCollapsed && (
-            <span className="text-white/60">Smooth Schedule</span>
+            <span className="text-white/60">{t('nav.smoothSchedule')}</span>
           )}
         </a>
         <button