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/identities.ts

@@ -0,0 +1,22 @@
+import type { Identity } from '@/lib/jmap/types';
+
+export function createDemoIdentities(): Identity[] {
+  return [
+    {
+      id: 'demo-identity-primary',
+      name: 'Demo User',
+      email: 'demo@example.com',
+      textSignature: 'Best regards,\nDemo User\nBulwark Mail Demo',
+      htmlSignature: '<p>Best regards,<br><b>Demo User</b><br>Bulwark Mail Demo</p>',
+      mayDelete: false,
+    },
+    {
+      id: 'demo-identity-alias',
+      name: 'Demo User',
+      email: 'demo+newsletter@example.com',
+      textSignature: '',
+      htmlSignature: '',
+      mayDelete: true,
+    },
+  ];
+}