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/lib/channelsUrlState.ts

@@ -17,7 +17,8 @@ export interface ChannelsUrlState {
   filters: FilterMap;
 }
 
-const KEYS = ["ctab", "csort", "cf"] as const;
+export const CHANNELS_KEYS = ["ctab", "csort", "cf"] as const;
+const KEYS = CHANNELS_KEYS;
 const VIEWS = ["subscribed", "discovery", "hidden", "blocked"];
 
 // Sort ⇄ `key:dir` (e.g. `subscribers:desc`) via the shared codec in columnSort. There is no default