feat: add "Forward as attachment" next to Export as .eml

Adds a "Forward as attachment" action to the message overflow menu
(desktop and mobile), right beside the existing "Export as .eml"
action. Opens a new forward-mode compose window with the original
message attached as a message/rfc822 file instead of quoted inline -
useful for reporting spam/phishing to an upstream gateway that expects
the raw original as an attachment (the primary motivating use case:
gateways like MxGuarddog require complete original headers, including
the full mail path, for scanning), or for preserving a message's exact
formatting/headers when forwarding.

Implementation reuses the composer's existing attachment-carry-forward
mechanism (the `attachments` useState initializer in
email-composer.tsx already carries a forwarded message's own
attachments into the new compose via `replyTo.attachments`) - this
just adds one synthetic entry representing the whole original message,
referenced by its existing blobId. No re-fetch or re-upload needed,
since JMAP blobs are account-scoped rather than per-email. The inline
quote-header step (prepareComposerQuoteHeader) is skipped, so the body
starts blank instead of quoting the original.

The core "build subject + attachment entry" logic is extracted into a
pure, unit-tested helper (lib/forward-as-attachment.ts) rather than
left inline in the already-large page component.

Adds the forward_as_attachment locale key to all 24 locales (English
text as a placeholder pending translation, following the existing
add-a-key convention) to satisfy the translations completeness test.

locales/ar/common.json

@@ -298,6 +298,7 @@
     "print": "طباعة",
     "view_source": "عرض المصدر",
     "export_email": "تصدير كملف ‎.eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "استيراد ملف ‎.eml أو ‎.zip",
     "keyboard_shortcuts": "اختصارات لوحة المفاتيح (؟)",
     "email_source": "مصدر الرسالة",

locales/ca/common.json

@@ -298,6 +298,7 @@
     "print": "Imprimeix",
     "view_source": "Mostra el codi font",
     "export_email": "Exporta com a .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importa .eml o .zip",
     "keyboard_shortcuts": "Dreceres de teclat (?)",
     "email_source": "Codi font del correu",

locales/cs/common.json

@@ -298,6 +298,7 @@
     "print": "Tisk",
     "view_source": "Zobrazit zdrojový kód",
     "export_email": "Exportovat jako .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importovat .eml nebo .zip",
     "keyboard_shortcuts": "Klávesové zkratky (?)",
     "email_source": "Zdrojový kód zprávy",

locales/da/common.json

@@ -298,6 +298,7 @@
     "print": "Udskriv",
     "view_source": "Vis kilde",
     "export_email": "Eksportér som .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importér .eml eller .zip",
     "keyboard_shortcuts": "Tastaturgenveje (?)",
     "email_source": "E-mail-kilde",

locales/de/common.json

@@ -298,6 +298,7 @@
     "print": "Drucken",
     "view_source": "Quelltext anzeigen",
     "export_email": "Als .eml exportieren",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": ".eml oder .zip importieren",
     "keyboard_shortcuts": "Tastaturkürzel (?)",
     "email_source": "E-Mail-Quelltext",

locales/en/common.json

@@ -298,6 +298,7 @@
     "print": "Print",
     "view_source": "View source",
     "export_email": "Export as .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Import .eml or .zip",
     "keyboard_shortcuts": "Keyboard shortcuts (?)",
     "email_source": "Email Source",

locales/es/common.json

@@ -298,6 +298,7 @@
     "print": "Imprimir",
     "view_source": "Ver código fuente",
     "export_email": "Exportar como .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importar .eml o .zip",
     "keyboard_shortcuts": "Atajos de teclado (?)",
     "email_source": "Código Fuente del Correo",

locales/fa/common.json

@@ -298,6 +298,7 @@
     "print": "چاپ",
     "view_source": "مشاهده منبع",
     "export_email": "خروجی .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "وارد کردن .eml یا .zip",
     "keyboard_shortcuts": "میانبرهای صفحه کلید (?)",
     "email_source": "منبع ایمیل",

locales/fr/common.json

@@ -298,6 +298,7 @@
     "print": "Imprimer",
     "view_source": "Voir la source",
     "export_email": "Exporter en .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importer .eml ou .zip",
     "keyboard_shortcuts": "Raccourcis clavier (?)",
     "email_source": "Source de l'email",

locales/he/common.json

@@ -245,6 +245,7 @@
     "print": "הדפס",
     "view_source": "צפה במקור",
     "export_email": "ייצא כ-.eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "ייבוא .eml",
     "keyboard_shortcuts": "קיצורי מקשים (?)",
     "email_source": "מקור דוא\"ל",

locales/hu/common.json

@@ -298,6 +298,7 @@
     "print": "Nyomtatás",
     "view_source": "Forrás megtekintése",
     "export_email": "Exportálás .eml-ként",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importálás .eml vagy .zip fájlból",
     "keyboard_shortcuts": "Billentyűparancsok (?)",
     "email_source": "E-mail forrás",

locales/it/common.json

@@ -298,6 +298,7 @@
     "print": "Stampa",
     "view_source": "Visualizza sorgente",
     "export_email": "Esporta come .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importa .eml o .zip",
     "keyboard_shortcuts": "Scorciatoie da tastiera (?)",
     "email_source": "Sorgente del messaggio",

locales/ja/common.json

@@ -298,6 +298,7 @@
     "print": "印刷",
     "view_source": "ソースを表示",
     "export_email": ".emlとしてエクスポート",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": ".eml または .zip をインポート",
     "keyboard_shortcuts": "キーボードショートカット (?)",
     "email_source": "メールソース",

locales/ko/common.json

@@ -298,6 +298,7 @@
     "print": "인쇄",
     "view_source": "원본 보기",
     "export_email": ".eml 파일로 내보내기",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": ".eml 또는 .zip 가져오기",
     "keyboard_shortcuts": "단축키 (?)",
     "email_source": "메일 원본",

locales/lv/common.json

@@ -298,6 +298,7 @@
     "print": "Drukāt",
     "view_source": "Skatīt avota kodu",
     "export_email": "Eksportēt kā .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importēt .eml vai .zip",
     "keyboard_shortcuts": "Īsinājumtaustiņi (?)",
     "email_source": "Vēstules avota kods",

locales/nl/common.json

@@ -298,6 +298,7 @@
     "print": "Afdrukken",
     "view_source": "Bron bekijken",
     "export_email": "Exporteren als .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": ".eml of .zip importeren",
     "keyboard_shortcuts": "Sneltoetsen (?)",
     "email_source": "E-mailbron",

locales/pl/common.json

@@ -298,6 +298,7 @@
     "print": "Drukuj",
     "view_source": "Pokaż źródło",
     "export_email": "Eksportuj jako .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importuj .eml lub .zip",
     "keyboard_shortcuts": "Skróty klawiszowe (?)",
     "email_source": "Źródło wiadomości",

locales/pt/common.json

@@ -298,6 +298,7 @@
     "print": "Imprimir",
     "view_source": "Ver código-fonte",
     "export_email": "Exportar como .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importar .eml ou .zip",
     "keyboard_shortcuts": "Atalhos de teclado (?)",
     "email_source": "Código-fonte do E-mail",

locales/ro/common.json

@@ -298,6 +298,7 @@
     "print": "Imprimare",
     "view_source": "Vizualizați sursa",
     "export_email": "Exportați ca fișier .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importați fișiere .eml sau .zip",
     "keyboard_shortcuts": "Comenzi rapide de la tastatură (?)",
     "email_source": "Sursa e-mailului",

locales/ru/common.json

@@ -298,6 +298,7 @@
     "print": "Распечатать",
     "view_source": "Просмотреть исходный код",
     "export_email": "Экспортировать как .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Импортировать .eml или .zip",
     "keyboard_shortcuts": "Сочетания клавиш (?)",
     "email_source": "Исходный код письма",

locales/sk/common.json

@@ -298,6 +298,7 @@
     "print": "Tlačiť",
     "view_source": "Zobraziť zdrojový kód",
     "export_email": "Exportovať ako .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Importovať .eml alebo .zip",
     "keyboard_shortcuts": "Klávesové skratky (?)",
     "email_source": "Zdrojový kód e-mailu",

locales/tr/common.json

@@ -298,6 +298,7 @@
     "print": "Yazdır",
     "view_source": "Kaynağı görüntüle",
     "export_email": ".eml olarak dışa aktar",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": ".eml veya .zip içe aktar",
     "keyboard_shortcuts": "Klavye kısayolları (?)",
     "email_source": "E-posta Kaynağı",

locales/uk/common.json

@@ -298,6 +298,7 @@
     "print": "Роздрукувати",
     "view_source": "Переглянути джерело",
     "export_email": "Експортувати як .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "Імпорт .eml або .zip",
     "keyboard_shortcuts": "Комбінації клавіш (?)",
     "email_source": "Джерело електронної пошти",

locales/zh/common.json

@@ -298,6 +298,7 @@
     "print": "打印",
     "view_source": "查看源码",
     "export_email": "导出为 .eml",
+    "forward_as_attachment": "Forward as attachment",
     "import_email": "导入 .eml 或 .zip",
     "keyboard_shortcuts": "键盘快捷键（？）",
     "email_source": "邮件源码",