feat(bookmarks): show who wrote each saved message

Saved-message cards showed the room and time but not the author, so in a busy
room you couldn't tell who said it without jumping. Now each card shows
"{sender} - {time ago}":

- Bookmark gains optional senderId/senderName (snapshotted at save time in
  Message.tsx from the already-computed sender display name); optional so
  existing stored bookmarks stay valid.
- The panel re-resolves the author's current display name live from the event
  when the room is joined, falling back to the stored snapshot for left rooms.
- Search now also matches the author name.

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

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

@@ -1190,6 +1190,8 @@ export const Message = React.memo(
                                       savedAt: Date.now(),
                                       previewText: body.slice(0, 120),
                                       roomName: room.name,
+                                      senderId,
+                                      senderName: senderDisplayName,
                                     });
                                   }
                                   closeMenu();