fix(bookmarks): drop unused senderId from stored bookmark

Review noted the 500-entry io.lotus.bookmarks blob approaches the ~65KB event
limit. The senderId I stored was never read back (the panel re-resolves the
live sender via the event and uses the senderName snapshot otherwise), so it
was dead payload. Keep only senderName.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/message/Message.tsx

@@ -1190,7 +1190,6 @@ export const Message = React.memo(
                                       savedAt: Date.now(),
                                       previewText: body.slice(0, 120),
                                       roomName: room.name,
-                                      senderId,
                                       senderName: senderDisplayName,
                                     });
                                   }