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.dom.test.tsx

@@ -167,6 +167,19 @@ describe("NavigationProvider · setPage's same-page guard", () => {
     expect(window.location.search).toBe("?page=plex&keep=1");
   });
 
+  it("leaves the keys of the modules it does not list by hand behind too: Files and the feed's", () => {
+    // Each module declares its keys (`urlKeys`), and the switch clears the union of them: a key
+    // missing from a hand-kept list rode along onto the next page's entry.
+    window.history.replaceState(
+      { sfPage: "files" },
+      "",
+      "/?page=files&folder=all&item=5&q=music&sort=oldest&keep=1",
+    );
+    const { result } = render();
+    act(() => result.current.setPage("plex"));
+    expect(window.location.search).toBe("?page=plex&keep=1");
+  });
+
   it("pushes nothing when it is already that page", () => {
     const { result } = render();
     act(() => result.current.setPage("plex"));
@@ -195,6 +208,25 @@ describe("NavigationProvider · setPage's same-page guard", () => {
   });
 });
 
+describe("NavigationProvider · a link carrying another page's keys", () => {
+  it("opens with only its own page's keys: what the page does not show, its address does not say", () => {
+    window.history.replaceState(
+      {},
+      "",
+      "/?page=activity&at=logs&folder=all&ctab=hidden&q=music&pscope=movie&pl=3&keep=1",
+    );
+    render();
+    expect(window.location.search).toBe("?page=activity&at=logs&keep=1");
+  });
+
+  it("keeps the channel page open over any page", () => {
+    window.history.replaceState({}, "", "/?page=stats&chan=UC1&folder=all");
+    const { result } = render();
+    expect(window.location.search).toBe("?page=stats&chan=UC1");
+    expect(result.current.channelView?.id).toBe("UC1");
+  });
+});
+
 describe("NavigationProvider · Back", () => {
   // A tab that had a since-removed module open, reloaded onto the new build: its older history
   // entries still name that page, and Back lands on one of them.

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,
     });

frontend/src/features/navigation/moduleRegistry.ts

@@ -48,6 +48,17 @@ export const REGISTERED_PAGES: readonly Page[] = ALL.map((m) => m.id);
 
 const BY_ID = Object.fromEntries(ALL.map((m) => [m.id, m])) as Record<Page, ModuleDef>;
 
+// Every module's address-bar keys, and those of every module but `page`: what a page switch clears and
+// what a page opens without (NavigationProvider). Derived from the definitions, so a new module's keys
+// are cleared the day it declares them.
+const ALL_URL_KEYS: readonly string[] = ALL.flatMap((m) => m.urlKeys);
+export function moduleUrlKeys(): readonly string[] {
+  return ALL_URL_KEYS;
+}
+export function foreignUrlKeys(page: Page): readonly string[] {
+  return ALL.filter((m) => m.id !== page).flatMap((m) => m.urlKeys);
+}
+
 // Look up a module's definition by page id (every Page is covered, incl. settings).
 export function moduleDef(id: Page): ModuleDef {
   return BY_ID[id];