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/stats/module.ts

@@ -10,4 +10,5 @@ export const statsModule: ModuleDef = {
   titleKey: "header.usageStats",
   icon: BarChart3,
   Page: lazyWithPreload(() => import("./Stats")),
+  urlKeys: [],
 };