fix(nav): the address follows the view through overlays, reloads and Back (review round 1)

- A URL mirror names the view again on every entry Back or Forward lands on, so a change made
  under a dialog, a drawer or a sub-view is not undone by the next reload.
- A view pushed from the entry a reload under a dialog started from no longer inherits its overlay
  mark; a reload under two overlays closes onto the page in one step.
- The linked player reopens over a live search, gives itself up when its list moves first, and
  plays alone over an emptied playlist; a capped Files list keeps the link it cannot resolve.
- The Feed press over a channel ends the search; leaving a search ends its asked mark; Files and
  Playlists searches end with their page; the feed always names its view and debounces its search.
- Search-box keys are declared per module and kept out of the client log below TRACE.
- Writers outside lib/ go through codecs; the URL contract scans computed keys, overlay writers and
  two-line reads, and holds every page to a bookmark round trip (Config, Activity, Playlists and the
  Files sort and viewer added).

frontend/src/lib/playlistsUrlState.ts

@@ -20,6 +20,15 @@ export function isStoredSort(s: SortState): boolean {
   return s?.key === PLAYLIST_STORED_SORT?.key && s?.dir === PLAYLIST_STORED_SORT?.dir;
 }
 
+/** The address-bar delta for the page's store: the playlist shown and the rail's search (each null
+ *  when not named). */
+export function playlistsParams(
+  selected: number | null,
+  search: string,
+): Record<string, string | null> {
+  return { pl: selected != null ? String(selected) : null, plq: search || null };
+}
+
 /** The address-bar delta for an open playlist's view; `null` clears it (no playlist open). */
 export function playlistViewParamUpdates(v: PlaylistView | null): Record<string, string | null> {
   if (!v) return { plsort: null, plf: null, plgroup: null };