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/types.ts

@@ -56,6 +56,8 @@ export interface Business {
   logoUrl?: string;
   emailLogoUrl?: string;
   logoDisplayMode?: 'logo-only' | 'text-only' | 'logo-and-text'; // How to display branding
+  timezone?: string; // IANA timezone (e.g., 'America/New_York')
+  timezoneDisplayMode?: 'business' | 'viewer'; // How times are displayed to users
   whitelabelEnabled: boolean;
   plan?: 'Free' | 'Professional' | 'Business' | 'Enterprise';
   status?: 'Active' | 'Suspended' | 'Trial';
@@ -68,6 +70,7 @@ export interface Business {
   initialSetupComplete?: boolean;
   customDomain?: string;
   customDomainVerified?: boolean;
+  bookingReturnUrl?: string; // URL to redirect customers after booking completion
   stripeConnectAccountId?: string;
   websitePages?: Record<string, { name: string; content: PageComponent[] }>;
   customerDashboardContent?: PageComponent[];