feat: add "Forward as attachment" to the message list context menu

Adds the same "Forward as attachment" action from the message viewer's
overflow menu to the right-click context menu on a message row in the
list, right after "Forward". Reuses the handleForwardAsAttachment
handler and buildForwardAsAttachmentPayload helper introduced earlier
in this PR - no new logic, just threading the prop down EmailList ->
EmailContextMenu the same way onForward already is.

blobId is already present on list-row emails (EMAIL_LIST_PROPERTIES
includes it specifically for the existing drag-out-to-filesystem .eml
export feature), so this works without any additional fetch. The menu
item is disabled if it's ever missing, matching how other actions
degrade when their handler prop isn't supplied.

Reuses the email_viewer.forward_as_attachment translation key already
added (via a second scoped useTranslations("email_viewer") call,
matching this file's existing cross-namespace pattern for
email_viewer.color_tag) rather than adding a duplicate key under
context_menu - avoids touching all 24 locale files again.

components/email/email-list.tsx

@@ -33,6 +33,7 @@ interface EmailListProps {
   onReply?: (email: Email) => void;
   onReplyAll?: (email: Email) => void;
   onForward?: (email: Email) => void;
+  onForwardAsAttachment?: (email: Email) => void;
   onMarkAsRead?: (email: Email, read: boolean) => void;
   onToggleStar?: (email: Email) => void;
   onTogglePinned?: (email: Email) => void;
@@ -63,6 +64,7 @@ export function EmailList({
   onReply,
   onReplyAll,
   onForward,
+  onForwardAsAttachment,
   onMarkAsRead,
   onToggleStar,
   onTogglePinned,
@@ -583,6 +585,7 @@ export function EmailList({
           onReply={() => onReply?.(contextMenu.data!)}
           onReplyAll={() => onReplyAll?.(contextMenu.data!)}
           onForward={() => onForward?.(contextMenu.data!)}
+          onForwardAsAttachment={() => onForwardAsAttachment?.(contextMenu.data!)}
           onMarkAsRead={(read) => onMarkAsRead?.(contextMenu.data!, read)}
           onToggleStar={() => onToggleStar?.(contextMenu.data!)}
           onTogglePinned={onTogglePinned ? () => onTogglePinned(contextMenu.data!) : undefined}