feat: add participant scheduling with iTIP invitations and inline calendar invitation banner

Add organizer/attendee UI with RSVP, contact autocomplete for participants,
scheduling messages, and inline calendar invitation banner in email viewer
with auto-detect .ics attachments, RSVP/import to calendar, and cancellation display.

lib/calendar-invitation.ts

@@ -0,0 +1,146 @@
+import type { Email, Attachment, CalendarEvent, CalendarParticipant } from '@/lib/jmap/types';
+
+export function findCalendarAttachment(email: Email): Attachment | null {
+  if (email.attachments) {
+    for (const att of email.attachments) {
+      if (
+        att.type === 'text/calendar' ||
+        att.type === 'application/ics' ||
+        att.name?.toLowerCase().endsWith('.ics') ||
+        att.name?.toLowerCase().endsWith('.ical')
+      ) {
+        return att;
+      }
+    }
+  }
+
+  if (email.textBody) {
+    for (const part of email.textBody) {
+      if (part.type === 'text/calendar' && part.blobId) {
+        return {
+          partId: part.partId,
+          blobId: part.blobId,
+          size: part.size,
+          name: part.name || 'invite.ics',
+          type: 'text/calendar',
+        };
+      }
+    }
+  }
+
+  return null;
+}
+
+export function getInvitationMethod(
+  event: Partial<CalendarEvent>
+): 'request' | 'reply' | 'cancel' | 'unknown' {
+  if (event.status === 'cancelled') {
+    return 'cancel';
+  }
+
+  if (event.participants && Object.keys(event.participants).length > 0) {
+    const hasOrganizer = Object.values(event.participants).some(
+      (p: CalendarParticipant) => p.roles?.owner || p.roles?.chair
+    );
+    if (hasOrganizer) {
+      return 'request';
+    }
+  }
+
+  return 'unknown';
+}
+
+export interface EventSummary {
+  title: string;
+  start: string | null;
+  end: string | null;
+  location: string | null;
+  organizer: string | null;
+  organizerEmail: string | null;
+  attendeeCount: number;
+}
+
+export function formatEventSummary(event: Partial<CalendarEvent>): EventSummary {
+  let location: string | null = null;
+  if (event.locations) {
+    const firstLocation = Object.values(event.locations)[0];
+    if (firstLocation?.name) {
+      location = firstLocation.name;
+    }
+  }
+
+  let organizer: string | null = null;
+  let organizerEmail: string | null = null;
+  let attendeeCount = 0;
+
+  if (event.participants) {
+    for (const p of Object.values(event.participants)) {
+      if (p.roles?.owner || p.roles?.chair) {
+        organizer = p.name || p.email || null;
+        organizerEmail = p.email || null;
+      }
+      if (p.roles?.attendee) {
+        attendeeCount++;
+      }
+    }
+  }
+
+  let end: string | null = null;
+  if (event.utcEnd) {
+    end = event.utcEnd;
+  } else if (event.start && event.duration) {
+    end = addDurationToDate(event.start, event.duration, event.timeZone);
+  }
+
+  return {
+    title: event.title || '',
+    start: event.utcStart || event.start || null,
+    end,
+    location,
+    organizer,
+    organizerEmail,
+    attendeeCount,
+  };
+}
+
+function addDurationToDate(start: string, duration: string, _timeZone?: string | null): string | null {
+  const match = duration.match(/^P(?:(\d+)D)?(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+)S)?)?$/);
+  if (!match) return null;
+
+  const days = parseInt(match[1] || '0');
+  const hours = parseInt(match[2] || '0');
+  const minutes = parseInt(match[3] || '0');
+  const seconds = parseInt(match[4] || '0');
+
+  const date = new Date(start);
+  if (isNaN(date.getTime())) return null;
+
+  date.setDate(date.getDate() + days);
+  date.setHours(date.getHours() + hours);
+  date.setMinutes(date.getMinutes() + minutes);
+  date.setSeconds(date.getSeconds() + seconds);
+
+  return date.toISOString();
+}
+
+export function findParticipantByEmail(
+  event: Partial<CalendarEvent>,
+  email: string
+): { id: string; participant: CalendarParticipant } | null {
+  if (!event.participants || !email) return null;
+
+  const lowerEmail = email.toLowerCase();
+  for (const [id, p] of Object.entries(event.participants)) {
+    if (p.email?.toLowerCase() === lowerEmail) {
+      return { id, participant: p };
+    }
+    if (p.sendTo) {
+      for (const addr of Object.values(p.sendTo)) {
+        if (addr.replace('mailto:', '').toLowerCase() === lowerEmail) {
+          return { id, participant: p };
+        }
+      }
+    }
+  }
+  return null;
+}