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/modules/feed/Feed.playerSession.dom.test.tsx

@@ -53,11 +53,13 @@ vi.mock("../../features/player/PlayerModal", async () => {
   return {
     default: function Player({
       video,
+      queue,
       onState,
       onWrite,
       onClose,
     }: {
       video: { id: string };
+      queue?: { id: string }[];
       onState: (id: string, status: string, v?: unknown) => void;
       onWrite?: (write: Promise<unknown>) => void;
       onClose: () => void;
@@ -71,6 +73,9 @@ vi.mock("../../features/player/PlayerModal", async () => {
       );
       return (
         <div>
+          <p>
+            playing {video.id} in {queue ? queue.map((v) => v.id).join(",") : "no list"}
+          </p>
           <button onClick={() => onState(video.id, "watched")}>mark</button>
           <button
             onClick={() =>
@@ -116,6 +121,7 @@ afterEach(() => {
   player.watchOnUnmount = false;
   player.flush = null;
   clearAll();
+  window.history.replaceState(null, "", "/");
 });
 
 // A cached related list (the player's, kept for the session) holding these videos.
@@ -190,6 +196,23 @@ function mount({
   return { qc, feed, summary };
 }
 
+// The URL is the view: a reload or a bookmark of the player reopens it once the list is in.
+describe("Feed — a player the address names", () => {
+  it("reopens on its video, the list as its queue", async () => {
+    window.history.replaceState(null, "", "/?watch=v1");
+    mount();
+    expect(await screen.findByText("playing v1 in v1")).toBeTruthy();
+  });
+
+  it("plays a video the loaded list does not hold alone, from the catalog", async () => {
+    window.history.replaceState(null, "", "/?watch=far");
+    const row = vi.spyOn(api, "videoRow").mockResolvedValue({ ...video("new"), id: "far" });
+    mount();
+    expect(await screen.findByText("playing far in no list")).toBeTruthy();
+    expect(row).toHaveBeenCalledWith("far");
+  });
+});
+
 describe("Feed — a video marked watched in the player", () => {
   it("stays out of an Unwatched view when the player writes its position into the cache", async () => {
     const { qc } = mount();

frontend/src/modules/feed/Feed.tsx

@@ -80,6 +80,8 @@ import ViewSwitcher from "../../components/ui/ViewSwitcher";
 import VirtualFeed from "./VirtualFeed";
 import SegmentedToggle from "../../components/ui/SegmentedToggle";
 import { IconButton } from "../../components/ui/IconButton";
+import { useOpenLinkedPlayer } from "../../features/player/useOpenLinkedPlayer";
+
 // Lazy: the in-app YouTube player (IFrame API) loads only when a video is first opened.
 const PlayerModal = lazy(() => import("../../features/player/PlayerModal"));
 
@@ -209,10 +211,13 @@ export default function Feed({
   // A reset's position 0 lives in the cached lists (see onResetState), where a refetch that set out
   // before the server took the reset puts the old position back: written again on its landing.
   const resets = useRef(new Map<string, number>());
-  // The open player: which video and where to start (null = resume from saved position).
-  const [activeVideo, setActiveVideo] = useState<{ video: Video; startAt: number | null } | null>(
-    null,
-  );
+  // The open player: which video and where to start (null = resume from saved position). `alone`:
+  // opened from an address naming a video this list has not loaded, so it plays without the queue.
+  const [activeVideo, setActiveVideo] = useState<{
+    video: Video;
+    startAt: number | null;
+    alone?: boolean;
+  } | null>(null);
   const qc = useQueryClient();
   const videoMarks = useVideoMarks();
   // Live search turned off on this instance: the empty feed offers no search that would be refused.
@@ -509,6 +514,18 @@ export default function Feed({
   // sequence (no reindex/reload mid-play), matching "watched doesn't affect the list". A video that
   // goes hidden still drops out ("visible affects").
   const playerQueue: Video[] = merged.filter((v) => v.status !== "hidden");
+  // A reload or a bookmark of the player reopens it once this list is in, the list as its queue again
+  // (the feed's filters are in the address under it). Not over a live search: that is not in the
+  // address, and a reload leaves it.
+  useOpenLinkedPlayer(
+    !ytActive && !query.isLoading,
+    (id) => playerQueue.find((v) => v.id === id),
+    (video, inList) => {
+      openVideo(video);
+      // Not in what this list has loaded (further down it, or no longer in it): it plays alone.
+      if (!inList) setActiveVideo((a) => a && { ...a, alone: true });
+    },
+  );
 
   // A live search ingests new catalog videos, so the disabled feed queries still hold their stale
   // pre-search cache. Drop them so the feed re-fetches fresh on return.
@@ -1006,7 +1023,7 @@ export default function Feed({
           <PlayerModal
             video={activeVideo.video}
             startAt={activeVideo.startAt}
-            queue={playerQueue}
+            queue={activeVideo.alone ? undefined : playerQueue}
             onClose={closePlayer}
             onState={onState}
             onWrite={trackSessionWrite}

frontend/src/modules/feed/module.ts

@@ -17,6 +17,7 @@ export const feedModule: ModuleDef = {
   Page: lazyWithPreloadAlongside(() => import("./FeedPage"), Rail),
   Store: FeedStore,
   urlKeys: FEED_KEYS,
+  playsVideos: true,
 };
 
 // The channel page: a feed scoped to one channel, opened over any page (App renders it in place of the