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/lib/moduleDef.ts

@@ -50,4 +50,8 @@ export type ModuleDef = {
   // along onto another's URL. Empty for a page with nothing of its own in the URL. Each key belongs to
   // one module and none is a shell key (moduleUrl.contract.test).
   urlKeys: readonly string[];
+  // Its page plays YouTube videos in the player and reopens it from an address that names one (a
+  // reload, a bookmark — lib/playerUrlState), with its own list as the queue. A page without it gets
+  // the shell's player for such an address: the video alone.
+  playsVideos?: boolean;
 };