fix(player): the resume position reaches the playlist page and live search, not just the feed

Deep-review R4. 1630a3df wrote the YT player's stored position into the react-query cache so a
reopen could not rewind it — but only the feed actually read it back:

- Playlists renders from its own undoable copy of the list, re-seeded only when the SET of ids
  changes, so the patched position never reached it: close a video at 830 s, reopen it from the
  playlist, and it resumed at the old spot and the next checkpoint wrote that over the 830. A detail
  with the same membership now rebases its fresh rows into every snapshot, keeping order and undo
  history. (The ids-only guard is older than this round; the write that depended on it was new.)
- Live YouTube search results carry the same per-user position and open the same player, but their
  cache was never patched. It is now — patched only, never invalidated, since refetching an
  API-sourced search page spends 100 quota units.

Tests for what had none: a Playlists harness (PlayerModal stubbed to record its launch props) and a
PlayerModal harness with a fake IFrame API that records where each item is started — the latter pins
that stepping back to an item watched earlier in the same session resumes at THIS session's position.
3 mutations, each red on its assertion.

frontend/src/components/Playlists.tsx

@@ -266,6 +266,14 @@ export default function Playlists() {
     if (key !== lastSetRef.current) {
       lastSetRef.current = key;
       order.reset(detail.items);
+    } else {
+      // Same membership, fresher ROWS — a watch position the player just stored, a status. The
+      // guard above looks at the id set only, so without this the list kept showing, and handing the
+      // player, each row as it was when the list first loaded: watch a video to 830 s, close it,
+      // reopen it, and it resumed at the old spot and the next checkpoint wrote that over the 830.
+      // `rebase` takes the fresh rows into every snapshot and keeps the order and its undo history.
+      const fresh = new Map(detail.items.map((v) => [v.id, v]));
+      order.rebase((list) => list.map((v) => fresh.get(v.id) ?? v));
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [detail]);