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.

locales/pt/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Marcar como Não Lido",
     "star": "Adicionar Estrela",
     "unstar": "Remover Estrela",
+    "pin": "Fixar",
+    "unpin": "Desafixar",
     "move_to": "Mover para...",
     "archive": "Arquivar",
     "delete": "Excluir",