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/lib/urlState.ts

@@ -5,6 +5,7 @@
 // retired. Only non-default values are emitted so URLs stay clean. NOTE: the browser TAB title stays
 // the short module/channel name (NavigationProvider owns document.title) — the query never leaks into it.
 import { TAG_CATEGORIES, type FeedFilters } from "./api";
+import { CHANNEL_PAGE_KEYS } from "./channelPageUrlState";
 
 export const FEED_KEYS = [
   "q",
@@ -31,12 +32,13 @@ export const FEED_KEYS = [
 ] as const;
 const KEYS = FEED_KEYS;
 
-// The keys the shell itself reads, whatever page is on screen: the page, the channel page (it opens
-// over any page), the welcome card's mode and the one-time entry links. No module may declare one of
+// The keys the shell itself reads, whatever page is on screen: the page, the channel page and its view
+// (it opens over any page, lib/channelPageUrlState), the welcome card's mode and the one-time entry links. No module may declare one of
 // these (moduleUrl.contract.test), and a page switch never clears them.
 export const SHELL_KEYS = [
   "page",
   "chan",
+  ...CHANNEL_PAGE_KEYS,
   "m",
   "link",
   "oauth",
@@ -228,6 +230,21 @@ export function urlWithParams(updates: Record<string, string | null>): string {
   return hrefHere(params);
 }
 
+/** The channel page's address, opened over the page on screen: the shell's own keys (the page, an
+ * entry link not yet read) and the channel — none of the page's keys, and none of another channel's
+ * view (the channel page writes its own, lib/channelPageUrlState). What `openChannel` pushes, and what a
+ * channel link points at, so a new tab opens the same view. */
+export function channelUrl(id: string): string {
+  const here = new URLSearchParams(window.location.search);
+  const viewKeys: readonly string[] = CHANNEL_PAGE_KEYS;
+  const params = new URLSearchParams();
+  for (const k of SHELL_KEYS) {
+    const v = k === "chan" ? id : here.get(k);
+    if (v != null && !viewKeys.includes(k)) params.set(k, v);
+  }
+  return hrefHere(params);
+}
+
 /** This page's address with its query replaced by `query` and its fragment by `hash` — what every
  * address-bar writer hands to `history`. Built on the URL itself, never by gluing `pathname` in front:
  * a path opened as `//evil.com/x` would read as a link to ANOTHER host, `history.replaceState` refuses