feat(nav): each module declares its URL keys; a page switch clears them and a page opens without another's

The shell cleared a hand-kept key list on a page switch (it missed the Files and feed keys) and cleared
nothing when a page opened from a link carrying another page's keys. ModuleDef.urlKeys now names every
key a page's view writes, the shell derives both from the registry, and moduleUrl.contract.test holds the
declarations true: one owner per key, no shell key, exactly what each codec writes, and no undeclared
key literal in the app.

frontend/src/modules/plex/module.ts

@@ -1,6 +1,7 @@
 import { Clapperboard } from "lucide-react";
 import { lazyWithPreload, lazyWithPreloadAlongside } from "../../lib/lazyWithPreload";
 import type { ModuleDef } from "../../lib/moduleDef";
+import { PLEX_KEYS, PLEX_SUB_KEY } from "../../lib/plexUrlState";
 import { PlexStore } from "./PlexStore";
 
 // The rail loads with the page (lazyWithPreloadAlongside), so it never arrives after it.
@@ -17,4 +18,5 @@ export const plexModule: ModuleDef = {
   Page: lazyWithPreloadAlongside(() => import("./PlexBrowse"), Rail),
   maxWidth: "100rem",
   Store: PlexStore,
+  urlKeys: [...PLEX_KEYS, PLEX_SUB_KEY],
 };