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.

app/(main)/[locale]/page.tsx

@@ -1652,6 +1652,45 @@ export default function Home() {
     }
   };
 
+  const handleTogglePinned = async (emailToPin: Email) => {
+    if (!client) return;
+
+    try {
+      const email = emails.find(e => e.id === emailToPin.id) ?? emailToPin;
+      const isPinned = email.keywords?.['$pinned'] === true;
+      // JMAP keywords are a set of present keys - drop the key to unpin
+      // rather than writing a false value.
+      const keywords = { ...email.keywords };
+      if (isPinned) {
+        delete keywords['$pinned'];
+      } else {
+        keywords['$pinned'] = true;
+      }
+
+      // Same unified-view routing as color tags: write to the email's own
+      // account via the login it is reachable through. (#281)
+      const pinClientId = isUnifiedView ? email.sourceClientAccountId : undefined;
+      const pinAccountId = isUnifiedView ? email.sourceAccountId : undefined;
+      const pinClient = pinClientId
+        ? (useAuthStore.getState().getClientForAccount(pinClientId) ?? client)
+        : client;
+
+      await pinClient.updateEmailKeywords(email.id, keywords, pinAccountId);
+
+      // Patch in place so the icon flips immediately, then refetch the first
+      // page so the mail floats/sinks per the server's pinned-first sort.
+      // Skip the refetch where that sort does not apply (unified views) or
+      // where it would replace a tag-filtered list (refreshCurrentMailbox
+      // fetches by folder only).
+      setEmailKeywordsLocal(email.id, keywords);
+      if (!isUnifiedView && !useEmailStore.getState().selectedKeyword) {
+        void refreshCurrentMailbox(client);
+      }
+    } catch (error) {
+      console.error("Failed to toggle pin:", error);
+    }
+  };
+
   const handleSetColorTag = async (emailId: string, color: string | null) => {
     if (!client) return;
 
@@ -3042,6 +3081,9 @@ export default function Home() {
                     await toggleStar(client, email.id);
                   }
                 }}
+                onTogglePinned={async (email) => {
+                  await handleTogglePinned(email);
+                }}
                 onDelete={async (email) => {
                   await handleDelete(email);
                 }}