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

locales/en/common.json

@@ -32,6 +32,7 @@
     "storage": "Storage",
     "sign_out": "Sign out",
     "contacts": "Contacts",
+    "calendar": "Calendar",
     "settings": "Settings",
     "loading_mailboxes": "Loading mailboxes...",
     "push_connected": "Real-time updates active",
@@ -375,7 +376,8 @@
       "account": "Account",
       "identities": "Identities",
       "vacation": "Vacation Responder",
-      "advanced": "Advanced"
+      "advanced": "Advanced",
+      "calendar": "Calendar"
     },
     "appearance": {
       "title": "Appearance",
@@ -902,6 +904,115 @@
       "error_delete": "Failed to delete contact"
     }
   },
+  "calendar": {
+    "title": "Calendar",
+    "back_to_email": "Back to email",
+    "my_calendars": "My calendars",
+    "views": {
+      "month": "Month",
+      "week": "Week",
+      "day": "Day",
+      "agenda": "Agenda",
+      "today": "Today"
+    },
+    "events": {
+      "create": "Create event",
+      "edit": "Edit event",
+      "delete": "Delete event",
+      "details": "Event details",
+      "no_events": "No events",
+      "all_day": "All day",
+      "more": "+{count} more",
+      "no_title": "(No title)",
+      "today_header": "Today",
+      "tomorrow_header": "Tomorrow"
+    },
+    "form": {
+      "title": "Title",
+      "description": "Description",
+      "location": "Location",
+      "start_date": "Start date",
+      "end_date": "End date",
+      "start_time": "Start time",
+      "end_time": "End time",
+      "all_day_event": "All-day event",
+      "calendar_select": "Calendar",
+      "save": "Save",
+      "cancel": "Cancel",
+      "delete_confirm": "Are you sure you want to delete this event?",
+      "color": "Color"
+    },
+    "participants": {
+      "title": "Participants",
+      "add": "Add participant",
+      "organizer": "Organizer",
+      "attendee": "Attendee",
+      "accepted": "Accepted",
+      "declined": "Declined",
+      "tentative": "Tentative",
+      "needs_action": "Needs action"
+    },
+    "recurrence": {
+      "title": "Recurrence",
+      "none": "Does not repeat",
+      "daily": "Daily",
+      "weekly": "Weekly",
+      "monthly": "Monthly",
+      "yearly": "Yearly",
+      "every_n_days": "Every {count} days",
+      "every_n_weeks": "Every {count} weeks",
+      "every_n_months": "Every {count} months",
+      "until": "Until",
+      "occurrences": "{count} occurrences"
+    },
+    "alerts": {
+      "title": "Reminder",
+      "none": "No reminder",
+      "at_time": "At time of event",
+      "minutes_before": "{count, plural, one {# minute before} other {# minutes before}}",
+      "hours_before": "{count, plural, one {# hour before} other {# hours before}}",
+      "days_before": "{count, plural, one {# day before} other {# days before}}"
+    },
+    "settings": {
+      "title": "Calendar settings",
+      "default_view": "Default view",
+      "week_starts_on": "Week starts on",
+      "time_format": "Time format",
+      "default_calendar": "Default calendar",
+      "default_reminder": "Default reminder",
+      "time_format_12h": "12-hour",
+      "time_format_24h": "24-hour"
+    },
+    "days": {
+      "monday": "Monday",
+      "tuesday": "Tuesday",
+      "wednesday": "Wednesday",
+      "thursday": "Thursday",
+      "friday": "Friday",
+      "saturday": "Saturday",
+      "sunday": "Sunday",
+      "mon": "Mon",
+      "tue": "Tue",
+      "wed": "Wed",
+      "thu": "Thu",
+      "fri": "Fri",
+      "sat": "Sat",
+      "sun": "Sun"
+    },
+    "notifications": {
+      "event_created": "Event created",
+      "event_updated": "Event updated",
+      "event_deleted": "Event deleted",
+      "calendar_created": "Calendar created",
+      "calendar_deleted": "Calendar deleted"
+    },
+    "status": {
+      "loading_calendars": "Loading calendars...",
+      "loading_events": "Loading events..."
+    },
+    "nav_prev": "Previous",
+    "nav_next": "Next"
+  },
   "advanced_search": {
     "title": "Advanced Search",
     "from": "From",