Feature: pin emails to the top of the folder list

Outlook-Web-style pinning: a context-menu Pin/Unpin action stores a
$pinned keyword on the message (plain IMAP-compatible flag, survives
other clients), and pinned mails stay at the top of the folder list
regardless of age, marked with a pin icon.

Ordering is done server-side via the hasKeyword sort comparator
(RFC 8621), applied consistently to the folder fetch, pagination and
the push-refresh so page windows stay stable. The client-side safety
sort in getEmails mirrors it, and sortThreadGroups keeps threads
containing a pinned mail on top so the client-side thread grouping
does not undo the order.

The new-mail notification in refreshCurrentMailbox now checks the
first non-pinned entry: with pinned mails on top, the newest mail is
no longer at index 0 and arrivals would never have notified.

The toggle reuses the color-tag pathway (routed keyword write for
unified views, in-place local patch), then refetches the first page
so the mail floats or sinks immediately. Search and unified views
keep their existing order.

Pin/Unpin strings are added to all 21 locales.

lib/demo/demo-client.ts

@@ -151,12 +151,16 @@ export class DemoJMAPClient implements IJMAPClient {
 
   // ── Emails ────────────────────────────────────────────────────
 
-  async getEmails(mailboxId?: string, _accountId?: string, limit: number = 50, position: number = 0): Promise<{ emails: Email[]; hasMore: boolean; total: number }> {
+  async getEmails(mailboxId?: string, _accountId?: string, limit: number = 50, position: number = 0, _hasKeyword?: string, pinnedFirst?: boolean): Promise<{ emails: Email[]; hasMore: boolean; total: number }> {
     let filtered = this.data.emails;
     if (mailboxId) {
       filtered = filtered.filter(e => e.mailboxIds[mailboxId]);
     }
-    filtered.sort((a, b) => new Date(b.receivedAt).getTime() - new Date(a.receivedAt).getTime());
+    const pinRank = (e: Email) => (pinnedFirst && e.keywords?.['$pinned'] ? 1 : 0);
+    filtered.sort((a, b) =>
+      pinRank(b) - pinRank(a) ||
+      new Date(b.receivedAt).getTime() - new Date(a.receivedAt).getTime()
+    );
     const total = filtered.length;
     const emails = filtered.slice(position, position + limit);
     return { emails, hasMore: position + limit < total, total };