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,
                                     });
                                   }

src/app/hooks/useBookmarks.ts

@@ -8,10 +8,11 @@ export type Bookmark = {
   savedAt: number;
   previewText: string;
   roomName: string;
-  // Author of the saved message. Optional for backward compatibility with
+  // Author display name, snapshotted at save time. Optional for backward
-  // bookmarks stored before attribution was added; the panel resolves a live
+  // compatibility with bookmarks stored before attribution was added; the panel
-  // name when the room is joined and falls back to these snapshots otherwise.
+  // re-resolves a live name when the room is joined and falls back to this
-  senderId?: string;
+  // snapshot otherwise. (We deliberately don't store the sender MXID — it's never
+  // read back and would only grow the 500-entry account-data blob.)
   senderName?: string;
 };