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.

app/api/config/route.ts

@@ -35,5 +35,6 @@ export async function GET() {
     loginImprintUrl: process.env.LOGIN_IMPRINT_URL || '',
     loginPrivacyPolicyUrl: process.env.LOGIN_PRIVACY_POLICY_URL || '',
     loginWebsiteUrl: process.env.LOGIN_WEBSITE_URL || '',
+    demoMode: process.env.DEMO_MODE === 'true',
   });
 }