refactor(channels): share useSortState across the manager + discovery (R13 S1)

ChannelsProvider and ChannelDiscovery each hand-rolled the same sort plumbing
(useState<SortState> + parseSortState + a persist key). Both now use the shared
useSortState hook. hydrateSort gains an empty-options rule: with no vocabulary
passed it parses without healing (these owners heal for display via
ColumnSortControl against the live columns), so the un-healed provider store is
preserved. plexSortKeys made module-private (knip).

Closes R13 S1: feed, Plex, and the two channel tables all sort through one
SortControl + one SortState model. Gate green: tsc/eslint(0 err)/prettier/knip
clean, vitest 94.

frontend/src/lib/columnSort.ts

@@ -78,16 +78,20 @@ export function healSort(
   return sort && options.some((o) => o.key === sort.key) ? sort : fallback;
 }
 
-/** Hydrate a persisted raw string into a healed `SortState`. Accepts the JSON `{key,dir}` shape and
- * tolerates a legacy bare-key string (older builds stored just the key), falling back otherwise. */
+/** Hydrate a persisted raw string into a `SortState`. Accepts the JSON `{key,dir}` shape and tolerates
+ * a legacy bare-key string (older builds stored just the key). When `options` is empty, the value is
+ * parsed but NOT healed — for a table/provider that owns no vocabulary itself and heals for display
+ * elsewhere (e.g. `ColumnSortControl`); a non-empty `options` heals an absent key to `fallback`. */
 export function hydrateSort(
   raw: string | null,
   options: SortOption[],
   fallback: SortState = null,
 ): SortState {
   if (!raw) return fallback;
+  const heal = (s: SortState): SortState =>
+    options.length === 0 ? (s ?? fallback) : healSort(s, options, fallback);
   try {
-    return healSort(parseSortState(JSON.parse(raw)), options, fallback);
+    return heal(parseSortState(JSON.parse(raw)));
   } catch {
     const opt = options.find((o) => o.key === raw);
     return opt ? { key: raw, dir: opt.defaultDir ?? "asc" } : fallback;