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/shell/App.tsx

@@ -29,6 +29,7 @@ import KeyboardHelp from "../components/ui/KeyboardHelp";
 import AppCommandPalette from "./AppCommandPalette";
 import DownloadIntake from "../features/downloads/DownloadIntake";
 import DownloadTray from "../features/downloads/DownloadTray";
+import { LinkedPlayer } from "../features/player/LinkedPlayer";
 import ErrorDialog from "./ErrorDialog";
 import VersionBanner from "./VersionBanner";
 import VerifyBanner from "./VerifyBanner";
@@ -187,6 +188,9 @@ export default function App() {
       <AppCommandPalette />
       {/* The one download dialog: the Files link box, a link pasted onto any page, the palette. */}
       <DownloadIntake />
+      {/* The player an address names over a page that plays none itself (a channel page plays its own,
+          through its feed); decided once, at boot. */}
+      <LinkedPlayer hosted={!!channelView || !!moduleDef(page).playsVideos} />
       {/* The debug recorder: its start dialog, and its pill docked under the toasts. */}
       {me && <DebugRecorder />}
       <Suspense fallback={null}>