feat: add demo data for emails, files, filters, identities, mailboxes, vacation responses, and JMAP client interface

- Created demo emails with various states (inbox, sent, drafts, trash, etc.) in `emails.ts`.
- Added demo file nodes representing directories and files in `files.ts`.
- Implemented demo Sieve capabilities and scripts in `filters.ts`.
- Defined demo identities for users in `identities.ts`.
- Established demo mailboxes with permissions and counts in `mailboxes.ts`.
- Created a demo vacation response in `vacation.ts`.
- Introduced a comprehensive JMAP client interface in `client-interface.ts` to standardize interactions with the JMAP API.

lib/demo/fixtures/vacation.ts

@@ -0,0 +1,13 @@
+import type { VacationResponse } from '@/lib/jmap/types';
+
+export function createDemoVacationResponse(): VacationResponse {
+  return {
+    id: 'singleton',
+    isEnabled: false,
+    fromDate: null,
+    toDate: null,
+    subject: 'Out of Office',
+    textBody: 'Thank you for your email. I am currently out of the office and will return on Monday. For urgent matters, please contact support@example.com.',
+    htmlBody: '<p>Thank you for your email. I am currently out of the office and will return on Monday.</p><p>For urgent matters, please contact <a href="mailto:support@example.com">support@example.com</a>.</p>',
+  };
+}