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/layouts/PlatformLayout.tsx

@@ -1,5 +1,5 @@
 import React, { useState, useRef } from 'react';
-import { Outlet } from 'react-router-dom';
+import { Outlet, useLocation } from 'react-router-dom';
 import { Moon, Sun, Globe, Menu } from 'lucide-react';
 import { User } from '../types';
 import PlatformSidebar from '../components/PlatformSidebar';
@@ -22,6 +22,10 @@ const PlatformLayout: React.FC<PlatformLayoutProps> = ({ user, darkMode, toggleT
   const [isMobileMenuOpen, setIsMobileMenuOpen] = useState(false);
   const [ticketModalId, setTicketModalId] = useState<string | null>(null);
   const mainContentRef = useRef<HTMLElement>(null);
+  const location = useLocation();
+
+  // Pages that need edge-to-edge rendering (no padding)
+  const noPaddingRoutes = ['/help/api-docs'];
 
   useScrollToTop(mainContentRef);
 
@@ -84,7 +88,7 @@ const PlatformLayout: React.FC<PlatformLayoutProps> = ({ user, darkMode, toggleT
           </div>
         </header>
 
-        <main ref={mainContentRef} className="flex-1 overflow-auto bg-gray-50 dark:bg-gray-900 p-8">
+        <main ref={mainContentRef} className={`flex-1 overflow-auto bg-gray-50 dark:bg-gray-900 ${noPaddingRoutes.includes(location.pathname) ? '' : 'p-8'}`}>
           <Outlet />
         </main>
       </div>