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.

lib/forward-as-attachment.ts

@@ -0,0 +1,44 @@
+import type { Email } from "@/lib/jmap/types";
+import { buildForwardSubject } from "@/lib/subject-prefix";
+import { emailExportFilename } from "@/lib/download-filename";
+
+export interface ForwardAsAttachmentEntry {
+  blobId: string;
+  name: string;
+  type: "message/rfc822";
+  size: number;
+}
+
+export interface ForwardAsAttachmentPayload {
+  subject: string;
+  attachment: ForwardAsAttachmentEntry;
+}
+
+/**
+ * Build the subject and synthetic attachment entry for forwarding a
+ * message as a message/rfc822 attachment instead of inline-quoted text
+ * (e.g. reporting spam to an upstream gateway that expects the raw
+ * original as an attachment, or preserving exact formatting/headers).
+ *
+ * Referenced by blobId, not re-uploaded - JMAP blobs are account-scoped,
+ * not per-email, so the same blobId a message already has can be attached
+ * to a brand new outgoing email directly.
+ *
+ * Returns null when the email has no blobId (nothing to reference).
+ */
+export function buildForwardAsAttachmentPayload(
+  email: Email,
+  forwardPrefix: string,
+): ForwardAsAttachmentPayload | null {
+  if (!email.blobId) return null;
+
+  return {
+    subject: buildForwardSubject(email.subject, forwardPrefix),
+    attachment: {
+      blobId: email.blobId,
+      name: emailExportFilename(email),
+      type: "message/rfc822",
+      size: email.size,
+    },
+  };
+}