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/activity/module.ts

@@ -1,6 +1,7 @@
 import { ScrollText } from "lucide-react";
 import { lazyWithPreload } from "../../lib/lazyWithPreload";
 import type { ModuleDef } from "../../lib/moduleDef";
+import { ACTIVITY_KEYS } from "../../lib/activityUrlState";
 
 // This page as the shell and the navigation know it (lib/moduleDef). Its page loads lazily.
 export const activityModule: ModuleDef = {
@@ -12,4 +13,5 @@ export const activityModule: ModuleDef = {
   system: true,
   gate: (me) => me.role === "admin",
   Page: lazyWithPreload(() => import("./Activity")),
+  urlKeys: ACTIVITY_KEYS,
 };