feat(channels): shareable manager view in the URL — tab+sort+column-filters (S8)

The Channels-manager content state now round-trips through the address bar,
so a manager view is bookmarkable/shareable — the S4 pattern applied to the
last content page the audit flagged.

- New lib/channelsUrlState.ts (channelsParamUpdates / channelsParamsToState /
  hasChannelsParams) under NAMESPACED keys ctab/csort/cf — channels-unique
  like ?v=/?pl/pscope, so no other provider reads or clears them and the
  feed's URL-stripping can't delete them. The mixed scalar/array FilterMap
  (sync/priority scalars + tags multi) rides in `cf` as a JSON blob so both
  shapes round-trip exactly. Only non-default values emit.
- ChannelsProvider mirrors {view, sort, filters} via useUrlMirror, page-gated
  on channels (off-page the keys clear; localStorage still holds the state and
  it re-mirrors on return). A shared link wins over stored state: readChannels
  UrlSeed captured once, applied before paint through the slice setters (so the
  account-pin re-hydrate sees them dirty and won't reload storage over the
  shared view), then persisted to the account once pinned so it survives a
  later plain reload — mirrors PlexProvider's seed/rehydrate. The name search
  stays ephemeral (out of the shared contract).
- NavigationProvider's page-switch nulls ctab/csort/cf alongside the plex keys
  so switching modules resets to the manager root.

Tests: channelsUrlState.dom.test.ts (9 round-trip) + ChannelsProvider.dom
seed/mirror (2); the existing tab-history test's beforeEach now resets the
query string too (the mirror writes it). FE vitest 503→514. FE-only.

frontend/src/lib/channelsUrlState.ts

@@ -0,0 +1,98 @@
+// Serialize the Channels-manager content state (active tab + column sort + the column-filter map) to/
+// from the address bar so a manager view is bookmarkable/shareable — the S8 analogue of the feed's
+// urlState and Plex's plexUrlState (S4). Keys are NAMESPACED (`ctab`/`csort`/`cf`) so they're
+// channels-unique (like `?v=`/`?pl`/`pscope`): no other provider ever reads or clears them, and the
+// feed's URL-stripping can't delete them out from under the manager. Only non-default values are
+// emitted, so URLs stay clean. The manager's name search is deliberately EPHEMERAL (like the feed/plex
+// rail search) and stays OUT of the shared contract.
+import { parseSortState, type SortState } from "./columnSort";
+import { type FilterMap } from "../components/tableHeader";
+
+// The content state S8 mirrors. `view` is the raw tab string (the provider clamps it); `sort` is the
+// raw provider SortState (healed against live columns only at the point of use), so the user's real
+// preference round-trips through the URL.
+export interface ChannelsUrlState {
+  view: string;
+  sort: SortState;
+  filters: FilterMap;
+}
+
+const KEYS = ["ctab", "csort", "cf"] as const;
+const VIEWS = ["subscribed", "discovery", "hidden", "blocked"];
+
+// Sort ⇄ `key:dir` (e.g. `subscribers:desc`). Column sort keys are simple words (no colon), so a plain
+// split is safe. There is no default sort (fallback null), so any sort emits.
+function encodeSort(s: SortState): string | null {
+  return s ? `${s.key}:${s.dir}` : null;
+}
+function decodeSort(raw: string): SortState {
+  const i = raw.indexOf(":");
+  const key = i === -1 ? raw : raw.slice(0, i);
+  const dir = i === -1 ? "asc" : raw.slice(i + 1);
+  return parseSortState({ key, dir });
+}
+
+// The FilterMap mixes scalar column filters (priority=select, name=text, the sync chip) with array ones
+// (tags=multi), so it's carried as a compact JSON blob rather than a nested query string — JSON round-
+// trips both shapes exactly (and future multi columns for free), and the values are opaque enum/id
+// tokens where a readable querystring buys little. Empty entries are dropped so a cleared funnel emits
+// no `cf`.
+function encodeFilters(map: FilterMap): string | null {
+  const clean: FilterMap = {};
+  for (const [k, v] of Object.entries(map)) {
+    if (Array.isArray(v) ? v.length : v) clean[k] = v;
+  }
+  return Object.keys(clean).length ? JSON.stringify(clean) : null;
+}
+// A URL blob is untrusted input: accept only a plain object of string / string[] entries, anything else
+// heals to empty (same discipline as useFilterState's storage hydrate).
+function decodeFilters(raw: string): FilterMap {
+  try {
+    const v = JSON.parse(raw);
+    if (!v || typeof v !== "object" || Array.isArray(v)) return {};
+    const out: FilterMap = {};
+    for (const [k, val] of Object.entries(v)) {
+      if (typeof val === "string") out[k] = val;
+      else if (Array.isArray(val) && val.every((x) => typeof x === "string"))
+        out[k] = val as string[];
+    }
+    return out;
+  } catch {
+    return {};
+  }
+}
+
+/** The address-bar delta for the manager content state: each namespaced key mapped to its emitted
+ * value, or null to delete it (so a control back at its default clears the key). Pass `null` to clear
+ * ALL manager keys — used when the app is off the Channels page (localStorage still holds the state; it
+ * re-mirrors on return). Feed the result into urlWithParams(). */
+export function channelsParamUpdates(
+  state: ChannelsUrlState | null,
+): Record<string, string | null> {
+  if (!state) return { ctab: null, csort: null, cf: null };
+  return {
+    ctab: state.view && state.view !== "subscribed" ? state.view : null,
+    csort: encodeSort(state.sort),
+    cf: encodeFilters(state.filters),
+  };
+}
+
+/** True if the query string carries any manager content key we recognize (a shared manager link). */
+export function hasChannelsParams(params: URLSearchParams): boolean {
+  return KEYS.some((k) => params.has(k));
+}
+
+/** Merge the manager content keys from the URL over a base state (the localStorage-seeded values); an
+ * absent or invalid key keeps the base value, so a partial link (only `?ctab=`) is honoured. */
+export function channelsParamsToState(
+  params: URLSearchParams,
+  base: ChannelsUrlState,
+): ChannelsUrlState {
+  const tab = params.get("ctab");
+  const csort = params.get("csort");
+  return {
+    view: tab && VIEWS.includes(tab) ? tab : base.view,
+    sort: csort != null ? (decodeSort(csort) ?? base.sort) : base.sort,
+    filters: params.has("cf") ? decodeFilters(params.get("cf") ?? "") : base.filters,
+  };
+}