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/cs/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Označit jako nepřečtené",
     "star": "Označit hvězdičkou",
     "unstar": "Odebrat hvězdičku",
+    "pin": "Připnout",
+    "unpin": "Odepnout",
     "move_to": "Přesunout do...",
     "archive": "Archivovat",
     "delete": "Odstranit",

locales/da/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Markér som ulæst",
     "star": "Stjernemarkér",
     "unstar": "Fjern stjerne",
+    "pin": "Fastgør",
+    "unpin": "Frigør",
     "move_to": "Flyt til...",
     "archive": "Arkivér",
     "delete": "Slet",

locales/de/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Als ungelesen markieren",
     "star": "Stern hinzufügen",
     "unstar": "Stern entfernen",
+    "pin": "Anheften",
+    "unpin": "Lösen",
     "move_to": "Verschieben nach...",
     "archive": "Archivieren",
     "delete": "Löschen",

locales/en/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Mark as Unread",
     "star": "Star",
     "unstar": "Unstar",
+    "pin": "Pin",
+    "unpin": "Unpin",
     "move_to": "Move to...",
     "archive": "Archive",
     "delete": "Delete",

locales/es/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Marcar como No Leído",
     "star": "Destacar",
     "unstar": "Quitar Destacado",
+    "pin": "Anclar",
+    "unpin": "Desanclar",
     "move_to": "Mover a...",
     "archive": "Archivar",
     "delete": "Eliminar",

locales/fa/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "علامت‌گذاری خوانده نشده",
     "star": "ستاره‌دار",
     "unstar": "حذف ستاره",
+    "pin": "سنجاق کردن",
+    "unpin": "برداشتن سنجاق",
     "move_to": "انتقال به...",
     "archive": "بایگانی",
     "delete": "حذف",

locales/fr/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Marquer comme non lu",
     "star": "Marquer comme favori",
     "unstar": "Retirer des favoris",
+    "pin": "Épingler",
+    "unpin": "Désépingler",
     "move_to": "Déplacer vers...",
     "archive": "Archiver",
     "delete": "Supprimer",

locales/hu/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Olvasatlannak jelölés",
     "star": "Csillagozás",
     "unstar": "Csillagozás megszüntetése",
+    "pin": "Rögzítés",
+    "unpin": "Rögzítés feloldása",
     "move_to": "Áthelyezés ide...",
     "archive": "Archiválás",
     "delete": "Törlés",

locales/it/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Segna come non letto",
     "star": "Aggiungi stella",
     "unstar": "Rimuovi stella",
+    "pin": "Fissa",
+    "unpin": "Non fissare più",
     "move_to": "Sposta in...",
     "archive": "Archivia",
     "delete": "Elimina",

locales/ja/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "未読にする",
     "star": "スターを付ける",
     "unstar": "スターを外す",
+    "pin": "ピン留め",
+    "unpin": "ピン留めを外す",
     "move_to": "移動...",
     "archive": "アーカイブ",
     "delete": "削除",

locales/ko/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "읽지 않은 상태로 표시",
     "star": "별표 달기",
     "unstar": "별표 해제",
+    "pin": "고정",
+    "unpin": "고정 해제",
     "move_to": "이동...",
     "archive": "보관",
     "delete": "삭제",

locales/lv/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Atzīmēt kā nelasītu",
     "star": "Pievienot zvaigznīti",
     "unstar": "Noņemt zvaigznīti",
+    "pin": "Piespraust",
+    "unpin": "Atspraust",
     "move_to": "Pārvietot uz...",
     "archive": "Arhivēt",
     "delete": "Dzēst",

locales/nl/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Markeren als ongelezen",
     "star": "Ster toevoegen",
     "unstar": "Ster verwijderen",
+    "pin": "Vastmaken",
+    "unpin": "Losmaken",
     "move_to": "Verplaatsen naar...",
     "archive": "Archiveren",
     "delete": "Verwijderen",

locales/pl/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Oznacz jako nieprzeczytane",
     "star": "Oznacz gwiazdką",
     "unstar": "Usuń gwiazdkę",
+    "pin": "Przypnij",
+    "unpin": "Odepnij",
     "move_to": "Przenieś do...",
     "archive": "Archiwizuj",
     "delete": "Usuń",

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",

locales/ro/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Marcați ca necitit",
     "star": "Stea",
     "unstar": "Anulează marcarea cu stea",
+    "pin": "Fixează",
+    "unpin": "Anulează fixarea",
     "move_to": "Mergi la...",
     "archive": "Arhivează",
     "delete": "Șterge",

locales/ru/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Отметить как непрочитанное",
     "star": "Добавить звёздочку",
     "unstar": "Убрать звёздочку",
+    "pin": "Закрепить",
+    "unpin": "Открепить",
     "move_to": "Переместить в...",
     "archive": "В архив",
     "delete": "Удалить",

locales/sk/common.json

@@ -1943,6 +1943,8 @@
     "mark_unread": "Označiť ako neprečítané",
     "star": "Hviezdička",
     "unstar": "Odstrániť hviezdičku",
+    "pin": "Pripnúť",
+    "unpin": "Odopnúť",
     "move_to": "Presunúť do...",
     "archive": "Archivovať",
     "delete": "Odstrániť",

locales/tr/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Okunmadı Olarak İşaretle",
     "star": "Yıldız Ekle",
     "unstar": "Yıldızı Kaldır",
+    "pin": "Sabitle",
+    "unpin": "Sabitlemeyi kaldır",
     "move_to": "Şuraya taşı...",
     "archive": "Arşivle",
     "delete": "Sil",

locales/uk/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "Позначити як непрочитане",
     "star": "зірка",
     "unstar": "Зняти зірочку",
+    "pin": "Закріпити",
+    "unpin": "Відкріпити",
     "move_to": "Перейти до...",
     "archive": "Архів",
     "delete": "Видалити",

locales/zh/common.json

@@ -1954,6 +1954,8 @@
     "mark_unread": "标记为未读",
     "star": "加星标",
     "unstar": "取消星标",
+    "pin": "固定",
+    "unpin": "取消固定",
     "move_to": "移动到…",
     "archive": "归档",
     "delete": "删除",