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/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