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/home/module.ts

@@ -11,4 +11,7 @@ export const homeModule: ModuleDef = {
   icon: Home,
   Page: lazyWithPreload(() => import("./Home")),
   maxWidth: "80rem",
+  // Nothing of its own: the question is an answer, not a setting — the same question draws other
+  // cards once what you have watched moves on.
+  urlKeys: [],
 };