feat(player): the YouTube player and the file viewer are named in the address

The player names what it plays in its own history entry (watch, wrel, wtab): a reload or a
bookmark reopens it on the page that hosts it, with that page's list as the queue, or alone
from the catalog (GET /api/videos/{id}/row); a page that hosts none gets the shell's. Back
lands on the page's address without it (useBackToClose's url; ui/Modal passes it on). The
Files viewer and player do the same (fview, fplay). The Plex and channel-manager views are
always written in full, as their fallback is remembered state.

frontend/src/features/player/LinkedPlayer.tsx

@@ -0,0 +1,40 @@
+import { lazy, Suspense, useState } from "react";
+import { useTranslation } from "react-i18next";
+import { useQueryClient } from "@tanstack/react-query";
+import { type Video, type VideoStatus } from "../../lib/api";
+import { qk } from "../../lib/queryKeys";
+import { notifyActionError } from "../../lib/actionErrors";
+import { useStableCallback } from "../../lib/useLatest";
+import { writeVideoState } from "../video/videoWrites";
+import { useOpenLinkedPlayer } from "./useOpenLinkedPlayer";
+
+const PlayerModal = lazy(() => import("./PlayerModal"));
+
+/** The shell's player, for an address that names one over a page that hosts none of its own (the
+ *  channel manager, where the watched-videos dialog played it; any page a link names): the video
+ *  alone. Decided at boot, once — `hosted` is whether the page on screen then opens it itself. */
+export function LinkedPlayer({ hosted }: { hosted: boolean }) {
+  const { t } = useTranslation();
+  const qc = useQueryClient();
+  const [own] = useState(!hosted);
+  const [video, setVideo] = useState<Video | null>(null);
+  useOpenLinkedPlayer(
+    own,
+    () => undefined,
+    (v) => setVideo(v),
+  );
+  const onState = useStableCallback((id: string, status: VideoStatus) => {
+    const write = writeVideoState(qc, id, status);
+    write
+      .then(() => qc.invalidateQueries({ queryKey: qk.feed() }))
+      .catch((e: unknown) => notifyActionError(e, t("errors.writeFailed.videoState")));
+    return write;
+  });
+  const close = useStableCallback(() => setVideo(null));
+  if (!video) return null;
+  return (
+    <Suspense fallback={null}>
+      <PlayerModal video={video} startAt={null} onClose={close} onState={onState} />
+    </Suspense>
+  );
+}