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/__tests__/thread-utils.test.ts

@@ -116,37 +116,47 @@ describe('groupEmailsByThread', () => {
 });
 
 describe('sortThreadGroups', () => {
+  const makeGroup = (threadId: string, receivedAt: string, hasPinned = false): ThreadGroup => ({
+    threadId,
+    emails: [makeEmail({ receivedAt })],
+    latestEmail: makeEmail({ receivedAt }),
+    participantNames: ['A'],
+    hasUnread: false,
+    hasStarred: false,
+    hasPinned,
+    hasAttachment: false,
+    hasAnswered: false,
+    hasForwarded: false,
+    emailCount: 1,
+  });
+
   it('sorts groups by latestEmail.receivedAt descending', () => {
-    const groups: ThreadGroup[] = [
-      {
-        threadId: 'old',
-        emails: [makeEmail({ receivedAt: '2024-01-01T00:00:00Z' })],
-        latestEmail: makeEmail({ receivedAt: '2024-01-01T00:00:00Z' }),
-        participantNames: ['A'],
-        hasUnread: false,
-        hasStarred: false,
-        hasAttachment: false,
-        hasAnswered: false,
-        hasForwarded: false,
-        emailCount: 1,
-      },
-      {
-        threadId: 'new',
-        emails: [makeEmail({ receivedAt: '2024-06-01T00:00:00Z' })],
-        latestEmail: makeEmail({ receivedAt: '2024-06-01T00:00:00Z' }),
-        participantNames: ['B'],
-        hasUnread: false,
-        hasStarred: false,
-        hasAttachment: false,
-        hasAnswered: false,
-        hasForwarded: false,
-        emailCount: 1,
-      },
+    const groups = [
+      makeGroup('old', '2024-01-01T00:00:00Z'),
+      makeGroup('new', '2024-06-01T00:00:00Z'),
     ];
     const sorted = sortThreadGroups(groups);
     expect(sorted[0].threadId).toBe('new');
     expect(sorted[1].threadId).toBe('old');
   });
+
+  it('keeps pinned threads on top regardless of date', () => {
+    const groups = [
+      makeGroup('newest', '2024-06-01T00:00:00Z'),
+      makeGroup('old-pinned', '2024-01-01T00:00:00Z', true),
+      makeGroup('mid', '2024-03-01T00:00:00Z'),
+    ];
+    const sorted = sortThreadGroups(groups);
+    expect(sorted.map(g => g.threadId)).toEqual(['old-pinned', 'newest', 'mid']);
+  });
+
+  it('detects hasPinned from the $pinned keyword', () => {
+    const emails = [
+      makeEmail({ id: 'e1', keywords: { $seen: true } }),
+      makeEmail({ id: 'e2', keywords: { $seen: true, '$pinned': true } }),
+    ];
+    expect(groupEmailsByThread(emails)[0].hasPinned).toBe(true);
+  });
 });
 
 describe('getThreadParticipants', () => {
@@ -188,6 +198,7 @@ describe('mergeThreadEmails', () => {
       participantNames: ['Alice'],
       hasUnread: false,
       hasStarred: false,
+      hasPinned: false,
       hasAttachment: false,
       hasAnswered: false,
       hasForwarded: false,
@@ -210,6 +221,7 @@ describe('mergeThreadEmails', () => {
       participantNames: ['Alice'],
       hasUnread: false,
       hasStarred: false,
+      hasPinned: false,
       hasAttachment: false,
       hasAnswered: false,
       hasForwarded: false,