fix: export isCalendarMimeType for use in email attachment filtering

Previously isCalendarMimeType was a module-private function in
lib/calendar-invitation.ts. Exporting it allows the email viewer
to reuse the same MIME type detection logic when filtering out
calendar attachments, avoiding duplication of the type set.

lib/calendar-invitation.ts

@@ -67,7 +67,7 @@ function parseContentType(value?: string | null): { mimeType: string; params: Re
   };
 }
 
-function isCalendarMimeType(value?: string | null): boolean {
+export function isCalendarMimeType(value?: string | null): boolean {
   const { mimeType } = parseContentType(value);
   return mimeType === 'text/calendar' || mimeType === 'application/ics' || mimeType === 'application/icalendar';
 }