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/pages/HelpApiDocs.tsx

@@ -1131,13 +1131,9 @@ my $response = $ua->get('${SANDBOX_URL}/services/',
               <AlertCircle size={20} className="text-yellow-600 dark:text-yellow-400 flex-shrink-0 mt-0.5" />
               <div>
                 <h3 className="text-sm font-semibold text-yellow-800 dark:text-yellow-200">
-                  No Test Tokens Found
+                  {t('help.api.noTestTokensFound')}
                 </h3>
-                <p className="text-sm text-yellow-700 dark:text-yellow-300 mt-1">
-                  Create a <strong>test/sandbox</strong> API token in your Settings to see personalized code examples with your actual token.
-                  Make sure to check the "Sandbox Mode" option when creating the token.
-                  The examples below use placeholder tokens.
-                </p>
+                <p className="text-sm text-yellow-700 dark:text-yellow-300 mt-1" dangerouslySetInnerHTML={{ __html: t('help.api.noTestTokensMessage') }} />
               </div>
             </div>
           </div>
@@ -1150,10 +1146,10 @@ my $response = $ua->get('${SANDBOX_URL}/services/',
               <AlertCircle size={20} className="text-red-600 dark:text-red-400 flex-shrink-0 mt-0.5" />
               <div>
                 <h3 className="text-sm font-semibold text-red-800 dark:text-red-200">
-                  Error Loading Tokens
+                  {t('help.api.errorLoadingTokens')}
                 </h3>
                 <p className="text-sm text-red-700 dark:text-red-300 mt-1">
-                  Failed to load API tokens. Please check your connection and try refreshing the page.
+                  {t('help.api.errorLoadingTokensMessage')}
                 </p>
               </div>
             </div>