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.

components/email/email-viewer.tsx

@@ -19,6 +19,7 @@ import {
   Reply,
   ReplyAll,
   Forward,
+  Paperclip,
   Trash2,
   Archive,
   Star,
@@ -109,6 +110,7 @@ interface EmailViewerProps {
   onReply?: (draftText?: string) => void;
   onReplyAll?: () => void;
   onForward?: () => void;
+  onForwardAsAttachment?: () => void;
   onDelete?: () => void;
   onArchive?: () => void;
   onToggleStar?: () => void;
@@ -621,6 +623,7 @@ export function EmailViewer({
   onReply,
   onReplyAll,
   onForward,
+  onForwardAsAttachment,
   onDelete,
   onArchive,
   onToggleStar,
@@ -3340,6 +3343,16 @@ export function EmailViewer({
                 </button>
               )}
               <div className="h-px bg-border my-1" />
+              {/* Forward as attachment */}
+              {onForwardAsAttachment && (
+                <button
+                  onClick={() => { onForwardAsAttachment(); setMoreMenuOpen(false); setMoreMenuSub(null); }}
+                  className="w-full px-3 py-1.5 text-sm text-start hover:bg-muted text-foreground flex items-center gap-2"
+                >
+                  <Paperclip className="w-4 h-4" />
+                  {t('forward_as_attachment')}
+                </button>
+              )}
               {/* Export email */}
               <button
                 onClick={() => { handleExportEmail(); setMoreMenuOpen(false); setMoreMenuSub(null); }}
@@ -3462,6 +3475,15 @@ export function EmailViewer({
                 </button>
               )}
               <div className="h-px bg-border my-1" />
+              {onForwardAsAttachment && (
+                <button
+                  onClick={() => { onForwardAsAttachment(); setMoreMenuOpen(false); }}
+                  className="w-full px-4 py-3 min-h-[44px] text-sm text-start hover:bg-muted text-foreground flex items-center gap-3"
+                >
+                  <Paperclip className="w-5 h-5" />
+                  {t('forward_as_attachment')}
+                </button>
+              )}
               <button
                 onClick={() => { handleExportEmail(); setMoreMenuOpen(false); }}
                 className="w-full px-4 py-3 min-h-[44px] text-sm text-start hover:bg-muted text-foreground flex items-center gap-3"