feat: Reorganize settings sidebar and add plan-based feature locking

- Add locked state to Plugins sidebar item with plan feature check
- Create Branding section in settings with Appearance, Email Templates, Custom Domains
- Split Domains page into Booking (URLs, redirects) and Custom Domains (BYOD, purchase)
- Add booking_return_url field to Tenant model for customer redirects
- Update SidebarItem component to support locked prop with lock icon
- Move Email Templates from main sidebar to Settings > Branding
- Add communication credits hooks and payment form updates
- Add timezone fields migration and various UI improvements

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

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

frontend/src/App.tsx

@@ -78,7 +78,8 @@ const SettingsLayout = React.lazy(() => import('./layouts/SettingsLayout'));
 const GeneralSettings = React.lazy(() => import('./pages/settings/GeneralSettings'));
 const BrandingSettings = React.lazy(() => import('./pages/settings/BrandingSettings'));
 const ResourceTypesSettings = React.lazy(() => import('./pages/settings/ResourceTypesSettings'));
-const DomainsSettings = React.lazy(() => import('./pages/settings/DomainsSettings'));
+const BookingSettings = React.lazy(() => import('./pages/settings/BookingSettings'));
+const CustomDomainsSettings = React.lazy(() => import('./pages/settings/CustomDomainsSettings'));
 const ApiSettings = React.lazy(() => import('./pages/settings/ApiSettings'));
 const AuthenticationSettings = React.lazy(() => import('./pages/settings/AuthenticationSettings'));
 const EmailSettings = React.lazy(() => import('./pages/settings/EmailSettings'));
@@ -700,7 +701,9 @@ const AppContent: React.FC = () => {
                 <Route path="general" element={<GeneralSettings />} />
                 <Route path="branding" element={<BrandingSettings />} />
                 <Route path="resource-types" element={<ResourceTypesSettings />} />
-                <Route path="domains" element={<DomainsSettings />} />
+                <Route path="booking" element={<BookingSettings />} />
+                <Route path="email-templates" element={<EmailTemplates />} />
+                <Route path="custom-domains" element={<CustomDomainsSettings />} />
                 <Route path="api" element={<ApiSettings />} />
                 <Route path="authentication" element={<AuthenticationSettings />} />
                 <Route path="email" element={<EmailSettings />} />