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/hooks/useBookmarks.ts

@@ -8,6 +8,11 @@ export type Bookmark = {
   savedAt: number;
   previewText: string;
   roomName: string;
+  // Author of the saved message. Optional for backward compatibility with
+  // bookmarks stored before attribution was added; the panel resolves a live
+  // name when the room is joined and falls back to these snapshots otherwise.
+  senderId?: string;
+  senderName?: string;
 };
 
 const BOOKMARKS_KEY = 'io.lotus.bookmarks';