fix(channel): the channel page's address names the channel and its own view, not the page under it

Opening a channel kept every key of the page under it (`?q=music&sort=oldest&chan=…`, a sort the
channel did not show), and the channel's own tab, sort and content types lived in its state alone, so a
reload or a bookmark lost them. Now the channel's address is `channelUrl`: the shell's keys and `chan`,
shared by `openChannel` and the channel link's href. The page writes its view (`chtab`, `chsort`,
`chtypes`, lib/channelPageUrlState) and reopens it; these are shell keys, cleared by a page switch and
by closing it in place. While a channel stands over a page, no module store writes its keys (`urlPage`);
Back finds them on the page's own entry. A link written by an older build opens without the page's keys.

frontend/src/features/navigation/NavigationProvider.tsx

@@ -2,7 +2,13 @@ import { useCallback, useEffect, useMemo, useState, type ReactNode } from "react
 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 {
+  channelUrl,
+  resolvePage,
+  retiredParams,
+  urlWithParams,
+  type Page,
+} from "../../lib/urlState";
 import { useDocumentObjectTitle } from "../../lib/documentTitle";
 import { getAccountRaw, setAccountRaw } from "../../lib/storage";
 import {
@@ -13,6 +19,7 @@ import {
   rememberNavPosition,
 } from "../../lib/history";
 import { foreignUrlKeys, moduleUrlKeys, pageTitleKey, reachablePage } from "./moduleRegistry";
+import { CHANNEL_PAGE_KEYS } from "../../lib/channelPageUrlState";
 import {
   channelViewFrom,
   loadInitialPage,
@@ -41,11 +48,20 @@ import {
 function pageRootParams(page: Page): Record<string, string | null> {
   return {
     page: page === "feed" ? null : page,
-    chan: null,
-    ...Object.fromEntries(moduleUrlKeys().map((k) => [k, null])),
+    ...closedChannelParams(),
+    ...nulls(moduleUrlKeys()),
   };
 }
 
+const nulls = (keys: readonly string[]) => Object.fromEntries(keys.map((k) => [k, null]));
+
+/** No channel page: neither `chan` nor its view's keys, which the page itself writes (ChannelPage) —
+ *  a closed channel leaves none behind. (An opened one gets its address from `channelUrl`.) */
+const closedChannelParams = (): Record<string, string | null> => ({
+  chan: null,
+  ...nulls(CHANNEL_PAGE_KEYS),
+});
+
 export function NavigationProvider({ children }: { children: ReactNode }) {
   const { t, i18n } = useTranslation();
   // Held here as well as in Settings: this is the instance mounted on every boot, so a value only
@@ -128,7 +144,9 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
   // inside one.
   const openChannel = useCallback((id: string, name?: string) => {
     closeOverlaysThenNavigate(() => {
-      const url = urlWithParams({ chan: id });
+      // The channel's address names the channel alone: the page under it keeps its own keys on its
+      // own entry, where Back finds them, and writes none while the channel stands over it (urlPage).
+      const url = channelUrl(id);
       if (window.history.state?._chan) {
         const st = window.history.state;
         window.history.replaceState({ ...st, _chan: id, _chanName: name ?? null }, "", url);
@@ -148,7 +166,7 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
       // entry's channel marks and the ?chan param.
       setChannelView(null);
       const { _chan: _id, _chanName: _name, ...st } = window.history.state ?? {};
-      window.history.replaceState(st, "", urlWithParams({ chan: null }));
+      window.history.replaceState(st, "", urlWithParams(closedChannelParams()));
     }
   }, []);
 
@@ -234,9 +252,11 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     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.
+    // opens without them, so its address names only what it shows — and a channel page open over it
+    // shows none of the page's own (an older build's channel links carried the feed's filters).
+    const shown = channelViewFrom(st, window.location.search);
     const url = urlWithParams({
-      ...Object.fromEntries(foreignUrlKeys(mountPage).map((k) => [k, null])),
+      ...nulls(shown ? moduleUrlKeys() : foreignUrlKeys(mountPage)),
       ...retiredParams(linked ?? (resolvePage(remembered) === mountPage ? remembered : null)),
       page: mountPage === "feed" ? null : mountPage,
     });