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/demo-utils.ts

@@ -0,0 +1,35 @@
+let demoIdCounter = 0;
+
+/** Generate a unique demo ID with the given prefix. */
+export function generateDemoId(prefix: string = 'demo'): string {
+  return `${prefix}-${Date.now()}-${++demoIdCounter}`;
+}
+
+/**
+ * Generate an ISO date string relative to "now".
+ * @param daysOffset — whole days from today
+ * @param hoursOffset — additional hours offset (default 0)
+ * @param minutesOffset — additional minutes offset (default 0)
+ */
+export function demoDate(daysOffset: number, hoursOffset: number = 0, minutesOffset: number = 0): string {
+  const d = new Date();
+  d.setDate(d.getDate() + daysOffset);
+  d.setHours(d.getHours() + hoursOffset, d.getMinutes() + minutesOffset, 0, 0);
+  return d.toISOString();
+}
+
+/**
+ * Generate a local date-time string (YYYY-MM-DDTHH:mm:ss) for JSCalendar "start" fields.
+ */
+export function demoISODate(daysOffset: number, hours: number = 0, minutes: number = 0): string {
+  const d = new Date();
+  d.setDate(d.getDate() + daysOffset);
+  d.setHours(hours, minutes, 0, 0);
+  const pad = (n: number) => String(n).padStart(2, '0');
+  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}:00`;
+}
+
+/** Deep clone fixture data so in-memory mutations don't corrupt originals. */
+export function cloneFixtures<T>(data: T): T {
+  return JSON.parse(JSON.stringify(data));
+}