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/tailwind.config.js

@@ -11,16 +11,16 @@ export default {
             },
             colors: {
                 brand: {
-                    50: '#eff6ff',
-                    100: '#dbeafe',
-                    200: '#bfdbfe',
-                    300: '#93c5fd',
-                    400: '#60a5fa',
-                    500: '#3b82f6',
-                    600: '#2563eb',
-                    700: '#1d4ed8',
-                    800: '#1e40af',
-                    900: '#1e3a8a',
+                    50: 'var(--color-brand-50, #eff6ff)',
+                    100: 'var(--color-brand-100, #dbeafe)',
+                    200: 'var(--color-brand-200, #bfdbfe)',
+                    300: 'var(--color-brand-300, #93c5fd)',
+                    400: 'var(--color-brand-400, #60a5fa)',
+                    500: 'var(--color-brand-500, #3b82f6)',
+                    600: 'var(--color-brand-600, #2563eb)',
+                    700: 'var(--color-brand-700, #1d4ed8)',
+                    800: 'var(--color-brand-800, #1e40af)',
+                    900: 'var(--color-brand-900, #1e3a8a)',
                 },
             },
         },