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/channels/module.ts

@@ -1,6 +1,7 @@
 import { Tv } from "lucide-react";
 import { lazyWithPreload } from "../../lib/lazyWithPreload";
 import type { ModuleDef } from "../../lib/moduleDef";
+import { CHANNELS_KEYS } from "../../lib/channelsUrlState";
 import { ChannelsProvider } from "./ChannelsProvider";
 
 // This page as the shell and the navigation know it (lib/moduleDef). Its page loads lazily.
@@ -13,4 +14,5 @@ export const channelsModule: ModuleDef = {
   Page: lazyWithPreload(() => import("./Channels")),
   maxWidth: "96rem",
   Store: ChannelsProvider,
+  urlKeys: CHANNELS_KEYS,
 };