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/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",