feat(player): a bookmark reopens every player as it was, once

The Plex player names the playlist it plays through (v=player:<id>:playlist:<pid>), so a bookmark
rebuilds the queue from the playlist's own order while the item is still in it.

Found in the browser and fixed:
- after a reload the shell's player opened a second one over Home and the feed: it read the catalog
  row the page had fetched (a disabled query still reads the shared cache); it now waits for ready.
- every reload under an open overlay left a dead history step: the first overlay to open takes over
  the entry the document was loaded on.
- the feed's address was implicit, the same as the start-page landing, so a reloaded or bookmarked
  feed opened as Home: the feed now names itself (page=feed) like every page.
- a linked file or video that is gone was said, but stayed in the address and was said again on
  every reload: its keys are taken off.

The overlay address is written in one place (the entry is named once it is current), and the player
keys only the codec reads are no longer exported.

frontend/src/lib/playerUrlState.ts

@@ -4,9 +4,9 @@
 // entry (lib/history's `useBackToClose`), so Back lands on the page as it was, without the player.
 // The list it plays from is the page's: the feed's filters, the open playlist (`pl`) — already named.
 // No position: the player resumes from the server's (`video_states`), which is where it was left.
-export const PLAYER_WATCH_KEY = "watch"; // the item of the list playing now
-export const PLAYER_RELATED_KEY = "wrel"; // a related video played in place over it
-export const PLAYER_PANEL_KEY = "wtab"; // the wide panel's tab, when it is not the description
+const PLAYER_WATCH_KEY = "watch"; // the item of the list playing now
+const PLAYER_RELATED_KEY = "wrel"; // a related video played in place over it
+const PLAYER_PANEL_KEY = "wtab"; // the wide panel's tab, when it is not the description
 export const PLAYER_KEYS = [PLAYER_WATCH_KEY, PLAYER_RELATED_KEY, PLAYER_PANEL_KEY] as const;
 
 export type PlayerPanelTab = "description" | "related";
@@ -24,7 +24,7 @@ export function playerParamUpdates(state: PlayerUrlState): Record<string, string
 }
 
 /** The player an address names, or null when it names none. */
-export function playerFrom(params: URLSearchParams): PlayerUrlState | null {
+function playerFrom(params: URLSearchParams): PlayerUrlState | null {
   const watch = params.get(PLAYER_WATCH_KEY);
   if (!watch) return null;
   return {