feat(nav): tabs, list views and name searches are named in the address

Settings, Configuration and Users name their tab (stab/cfgtab/utab); a link's tab wins and a tab pre-selected
while its page is on screen now switches it. A playlist's sort, filters and grouping (plsort/plf/plgroup), the
Files sort (fsort, always written) and the name searches of Channels, Files, Playlists and Plex (cq/fq/plq/pq+pqf)
reopen from the address; the searches are written once the typing pauses.

frontend/src/lib/playlistsUrlState.ts

@@ -0,0 +1,43 @@
+import { decodeSortParam, encodeSortParam, type SortState } from "./columnSort";
+import { decodeFilterMap, encodeFilterMap } from "./filterMapParam";
+import { type FilterMap } from "./tableColumns";
+
+// The Playlists page's address-bar keys: the playlist shown (`pl`), the rail's name search (`plq`), and
+// the open playlist's view — its track sort, column filters and grouping (`plsort`/`plf`/`plgroup`).
+// The playlist is remembered per account too (PlaylistsProvider); the search and the view are
+// remembered nowhere else (the view starts over on every playlist), so each of those is written only
+// when it differs from the default: its key's absence IS the default.
+export const PLAYLISTS_KEYS = ["pl", "plq", "plsort", "plf", "plgroup"] as const;
+
+export type PlaylistView = { sort: SortState; filters: FilterMap; groupBy: boolean };
+
+// The neutral view: "#" ascending IS the stored order, so the header can show a real arrow for it
+// instead of the list looking unsorted (or, worse, looking sorted by whatever column the stored
+// order happens to correlate with — the user read a channel-grouped playlist as sorted by Channel).
+export const PLAYLIST_STORED_SORT: SortState = { key: "pos", dir: "asc" };
+
+export function isStoredSort(s: SortState): boolean {
+  return s?.key === PLAYLIST_STORED_SORT?.key && s?.dir === PLAYLIST_STORED_SORT?.dir;
+}
+
+/** 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 };
+  return {
+    // An unsorted view (a header clicked through to "none") is the stored order too.
+    plsort: v.sort && !isStoredSort(v.sort) ? encodeSortParam(v.sort) : null,
+    plf: encodeFilterMap(v.filters),
+    plgroup: v.groupBy ? "1" : null,
+  };
+}
+
+/** The view an address names; what it leaves out is the default. */
+export function playlistViewFrom(params: URLSearchParams): PlaylistView {
+  const sort = params.get("plsort");
+  const filters = params.get("plf");
+  return {
+    sort: (sort != null && decodeSortParam(sort)) || PLAYLIST_STORED_SORT,
+    filters: filters != null ? decodeFilterMap(filters) : {},
+    groupBy: params.get("plgroup") === "1",
+  };
+}