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.

components/email/email-context-menu.tsx

@@ -17,6 +17,8 @@ import {
   Mail,
   MailOpen,
   Star,
+  Pin,
+  PinOff,
   Trash2,
   Archive,
   FolderInput,
@@ -59,6 +61,7 @@ interface EmailContextMenuProps {
   onForward?: () => void;
   onMarkAsRead?: (read: boolean) => void;
   onToggleStar?: () => void;
+  onTogglePinned?: () => void;
   onDelete?: () => void;
   onArchive?: () => void;
   onSetColorTag?: (color: string | null) => void;
@@ -126,6 +129,7 @@ export function EmailContextMenu({
   onForward,
   onMarkAsRead,
   onToggleStar,
+  onTogglePinned,
   onDelete,
   onArchive,
   onSetColorTag,
@@ -149,6 +153,7 @@ export function EmailContextMenu({
   const emailKeywords = useSettingsStore((state) => state.emailKeywords);
   const isUnread = !email.keywords?.$seen;
   const isStarred = email.keywords?.$flagged;
+  const isPinned = email.keywords?.['$pinned'] === true;
   const isDraft = email.keywords?.['$draft'] === true;
   const currentColors = getCurrentColors(email.keywords);
   const showBatchActions = isMultiSelect && selectedCount > 1;
@@ -352,6 +357,15 @@ export function EmailContextMenu({
         />
       )}
 
+      {/* Pin/Unpin - only for single email; pinned mails float to the top of the list */}
+      {!showBatchActions && onTogglePinned && (
+        <ContextMenuItem
+          icon={isPinned ? PinOff : Pin}
+          label={isPinned ? t("unpin") : t("pin")}
+          onClick={() => handleAction(onTogglePinned)}
+        />
+      )}
+
       {/* Set tag submenu - only for single email */}
       {!showBatchActions && (
         <ContextMenuSubMenu icon={Tag} label={t("color_tag")}>