fix: hide "Forward as attachment" in overflow menu when blobId missing

The overflow menu ("...") showed "Forward as attachment" whenever the
handler was provided, regardless of whether the open email has a
blobId. If it doesn't, handleForwardAsAttachment immediately no-ops
(buildForwardAsAttachmentPayload returns null), so the item was
clickable but did nothing - inconsistent with the list context menu's
version, which is already disabled in that case
(!onForwardAsAttachment || !email.blobId).

Gate both occurrences (desktop and mobile layouts) on email?.blobId
too, matching the context menu's behavior.

Caught by GitHub Copilot's automated PR review.

components/email/email-viewer.tsx

@@ -3344,7 +3344,7 @@ export function EmailViewer({
               )}
               <div className="h-px bg-border my-1" />
               {/* Forward as attachment */}
-              {onForwardAsAttachment && (
+              {onForwardAsAttachment && email?.blobId && (
                 <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"
@@ -3475,7 +3475,7 @@ export function EmailViewer({
                 </button>
               )}
               <div className="h-px bg-border my-1" />
-              {onForwardAsAttachment && (
+              {onForwardAsAttachment && email?.blobId && (
                 <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"