feat: add JMAP Calendar integration with month/week/day/agenda views

Full calendar support via JMAP Calendars (RFC 8984):
- Event create/edit/delete with recurrence rules and reminders
- Multi-day event spanning, column-based overlap layout
- Locale-aware date formatting, first day of week and time format settings
- Real-time updates via push notifications
- ARIA accessibility, input validation, color sanitization
- Keyboard shortcuts, mobile touch targets, focus trap
- ICU pluralization for all 8 supported languages

app/[locale]/settings/page.tsx

@@ -10,11 +10,12 @@ import { EmailSettings } from '@/components/settings/email-settings';
 import { AccountSettings } from '@/components/settings/account-settings';
 import { IdentitySettings } from '@/components/settings/identity-settings';
 import { VacationSettings } from '@/components/settings/vacation-settings';
+import { CalendarSettings } from '@/components/settings/calendar-settings';
 import { AdvancedSettings } from '@/components/settings/advanced-settings';
 import { useAuthStore } from '@/stores/auth-store';
 import { cn } from '@/lib/utils';
 
-type Tab = 'appearance' | 'email' | 'account' | 'identities' | 'vacation' | 'advanced';
+type Tab = 'appearance' | 'email' | 'account' | 'identities' | 'vacation' | 'calendar' | 'advanced';
 
 export default function SettingsPage() {
   const router = useRouter();
@@ -23,6 +24,7 @@ export default function SettingsPage() {
   const [activeTab, setActiveTab] = useState<Tab>('appearance');
 
   const supportsVacation = client?.supportsVacationResponse() ?? false;
+  const supportsCalendar = client?.supportsCalendars() ?? false;
 
   const tabs: { id: Tab; label: string }[] = [
     { id: 'appearance', label: t('tabs.appearance') },
@@ -30,6 +32,7 @@ export default function SettingsPage() {
     { id: 'account', label: t('tabs.account') },
     { id: 'identities', label: t('tabs.identities') },
     ...(supportsVacation ? [{ id: 'vacation' as Tab, label: t('tabs.vacation') }] : []),
+    ...(supportsCalendar ? [{ id: 'calendar' as Tab, label: t('tabs.calendar') }] : []),
     { id: 'advanced', label: t('tabs.advanced') },
   ];
 
@@ -89,6 +92,7 @@ export default function SettingsPage() {
             {activeTab === 'account' && <AccountSettings />}
             {activeTab === 'identities' && <IdentitySettings />}
             {activeTab === 'vacation' && <VacationSettings />}
+            {activeTab === 'calendar' && <CalendarSettings />}
             {activeTab === 'advanced' && <AdvancedSettings />}
           </div>
         </div>