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/urlState.ts

@@ -6,6 +6,7 @@
 // the short module/channel name (NavigationProvider owns document.title) — the query never leaks into it.
 import { TAG_CATEGORIES, type FeedFilters } from "./api";
 import { CHANNEL_PAGE_KEYS } from "./channelPageUrlState";
+import { PLAYER_KEYS } from "./playerUrlState";
 
 export const FEED_KEYS = [
   "q",
@@ -33,12 +34,14 @@ export const FEED_KEYS = [
 const KEYS = FEED_KEYS;
 
 // The keys the shell itself reads, whatever page is on screen: the page, the channel page and its view
-// (it opens over any page, lib/channelPageUrlState), the welcome card's mode and the one-time entry links. No module may declare one of
-// these (moduleUrl.contract.test), and a page switch never clears them.
+// (it opens over any page, lib/channelPageUrlState), the YouTube player (the same, lib/playerUrlState),
+// the welcome card's mode and the one-time entry links. No module may declare one of these
+// (moduleUrl.contract.test); a page switch clears only the channel's and the player's.
 export const SHELL_KEYS = [
   "page",
   "chan",
   ...CHANNEL_PAGE_KEYS,
+  ...PLAYER_KEYS,
   "m",
   "link",
   "oauth",
@@ -231,12 +234,13 @@ export function urlWithParams(updates: Record<string, string | null>): string {
 }
 
 /** The channel page's address, opened over the page on screen: the shell's own keys (the page, an
- * entry link not yet read) and the channel — none of the page's keys, and none of another channel's
- * view (the channel page writes its own, lib/channelPageUrlState). What `openChannel` pushes, and what a
+ * entry link not yet read) and the channel — none of the page's keys, none of another channel's view
+ * (the channel page writes its own, lib/channelPageUrlState), and no player (opening a channel closes
+ * it). What `openChannel` pushes, and what a
  * channel link points at, so a new tab opens the same view. */
 export function channelUrl(id: string): string {
   const here = new URLSearchParams(window.location.search);
-  const viewKeys: readonly string[] = CHANNEL_PAGE_KEYS;
+  const viewKeys: readonly string[] = [...CHANNEL_PAGE_KEYS, ...PLAYER_KEYS];
   const params = new URLSearchParams();
   for (const k of SHELL_KEYS) {
     const v = k === "chan" ? id : here.get(k);