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/files/Files.menu.dom.test.tsx

@@ -745,6 +745,51 @@ describe("Files · the viewer", () => {
   });
 });
 
+// The URL is the view: the file open over the list is named in the address, so a reload or a bookmark
+// reopens it once the list is in.
+describe("Files · a file the address names", () => {
+  const doc = (id: number) =>
+    file(id, {
+      job_kind: "upload",
+      display_name: `Doc ${id}`,
+      asset: { media_kind: "file", preview: "pdf", container: "pdf", size_bytes: 10 },
+    } as never);
+  const here = () => new URLSearchParams(window.location.search);
+
+  it("reopens the viewer on it, and the viewer names the file it turns to", async () => {
+    window.history.replaceState(null, "", "/?page=files&fview=2");
+    mocks.files = [doc(2), doc(3)];
+    mocks.folders = [YT];
+    mount();
+    const dialog = await screen.findByRole("dialog");
+    expect(dialog.querySelector("iframe")!.getAttribute("src")).toBe(api.previewUrl(2));
+    await waitFor(() => expect(here().get("fview")).toBe("2"));
+    fireEvent.keyDown(window, { key: "ArrowRight" });
+    await waitFor(() => expect(here().get("fview")).toBe("3"));
+  });
+
+  it("reopens the player on a media file", async () => {
+    window.history.replaceState(null, "", "/?page=files&fplay=1");
+    mocks.files = [file(1)];
+    mocks.folders = [YT];
+    mount();
+    expect(await screen.findByText("playing 1")).toBeTruthy();
+  });
+
+  it("says so when the list no longer holds it", async () => {
+    window.history.replaceState(null, "", "/?page=files&fview=99");
+    mocks.files = [doc(2)];
+    mocks.folders = [YT];
+    mount();
+    await waitFor(() =>
+      expect(mocks.notify).toHaveBeenCalledWith(
+        expect.objectContaining({ level: "warning", message: "files.linkedGone" }),
+      ),
+    );
+    expect(screen.queryByRole("dialog")).toBeNull();
+  });
+});
+
 describe("Files · the view", () => {
   it("starts the list at the scroller's top, so its sticky header does not jump on the first scroll", () => {
     mocks.files = [file(2)];