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/features/navigation/NavigationProvider.tsx

@@ -3,7 +3,6 @@ import { useLatest } from "../../lib/useLatest";
 import { useTranslation } from "react-i18next";
 import { useMe } from "../../lib/useMe";
 import { resolvePage, retiredParams, urlWithParams, type Page } from "../../lib/urlState";
-import { ACTIVITY_KEYS } from "../../lib/activityUrlState";
 import { useDocumentObjectTitle } from "../../lib/documentTitle";
 import { getAccountRaw, setAccountRaw } from "../../lib/storage";
 import {
@@ -13,7 +12,7 @@ import {
   notifyNav,
   rememberNavPosition,
 } from "../../lib/history";
-import { pageTitleKey, reachablePage } from "./moduleRegistry";
+import { foreignUrlKeys, moduleUrlKeys, pageTitleKey, reachablePage } from "./moduleRegistry";
 import {
   channelViewFrom,
   loadInitialPage,
@@ -37,21 +36,13 @@ import {
 //
 // The contexts, their value types and the hooks live in navigationContext.ts.
 
-/** The address bar of `page`'s root: the page (feed implicit) with every view-specific key cleared. */
+/** The address bar of `page`'s root: the page (feed implicit) with every module's own keys cleared — a
+ *  page switch resets to that module's root, and its store mirrors what it holds back in. */
 function pageRootParams(page: Page): Record<string, string | null> {
   return {
     page: page === "feed" ? null : page,
     chan: null,
-    v: null,
-    pl: null,
-    pscope: null,
-    psort: null,
-    pshow: null,
-    pf: null,
-    ctab: null,
-    csort: null,
-    cf: null,
-    ...Object.fromEntries(ACTIVITY_KEYS.map((k) => [k, null])),
+    ...Object.fromEntries(moduleUrlKeys().map((k) => [k, null])),
   };
 }
 
@@ -116,12 +107,10 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
         setAccountRaw(PAGE_KEY, next);
         // Push an in-app history entry so browser/mouse Back steps through pages instead of leaving
         // the app. A fresh { sfPage } (no carried-over _yt/_chan markers) so each page starts at its
-        // root, and the address bar reflects the page (feed is implicit) while clearing the
-        // view-specific params (chan/v/pl and the plex content keys pscope/psort/pshow/pf) — a page
-        // switch resets to that module's root. Feed filters (owned by FeedFiltersProvider) are left
-        // intact; the plex/channels keys re-mirror from PlexProvider / ChannelsProvider on return to
-        // their page. The item it was opened from is still focused: the state setters above have not
-        // re-rendered anything yet.
+        // root, and the address bar reflects the page (feed is implicit) while clearing `chan` and
+        // every module's own keys (pageRootParams) — a page switch resets to that module's root, and
+        // each module's store mirrors its state back in on its page. The item it was opened from is
+        // still focused: the state setters above have not re-rendered anything yet.
         rememberNavPosition();
         window.history.pushState(
           { sfPage: next, sfScroll: nextNavId() },
@@ -244,7 +233,10 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     // part of the page that replaced it (an `at` tab).
     const linked = new URLSearchParams(window.location.search).get("page");
     const remembered = getAccountRaw(PAGE_KEY);
+    // A link can carry another page's keys (hand-edited, or written by an older build): the page
+    // opens without them, so its address names only what it shows.
     const url = urlWithParams({
+      ...Object.fromEntries(foreignUrlKeys(mountPage).map((k) => [k, null])),
       ...retiredParams(linked ?? (resolvePage(remembered) === mountPage ? remembered : null)),
       page: mountPage === "feed" ? null : mountPage,
     });