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/features/navigation/NavigationProvider.tsx

@@ -20,6 +20,7 @@ import {
 } from "../../lib/history";
 import { foreignUrlKeys, moduleUrlKeys, pageTitleKey, reachablePage } from "./moduleRegistry";
 import { CHANNEL_PAGE_KEYS } from "../../lib/channelPageUrlState";
+import { PLAYER_KEYS } from "../../lib/playerUrlState";
 import {
   channelViewFrom,
   loadInitialPage,
@@ -43,12 +44,14 @@ import {
 //
 // The contexts, their value types and the hooks live in navigationContext.ts.
 
-/** The address bar of `page`'s root: the page (feed implicit) with every module's own keys cleared — a
- *  page switch resets to that module's root, and its store mirrors what it holds back in. */
+/** The address bar of `page`'s root: the page (feed implicit), no channel or player over it, and every
+ *  module's own keys cleared — a page switch resets to that module's root, and its store mirrors what
+ *  it holds back in. */
 function pageRootParams(page: Page): Record<string, string | null> {
   return {
     page: page === "feed" ? null : page,
     ...closedChannelParams(),
+    ...nulls(PLAYER_KEYS),
     ...nulls(moduleUrlKeys()),
   };
 }