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/components/ChannelDiscovery.tsx

@@ -1,12 +1,13 @@
-import { useEffect, useState } from "react";
+import { useEffect } from "react";
 import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { UserPlus } from "lucide-react";
 import { api, type DiscoveredChannel } from "../lib/api";
-import { accountKey, LS, readAccount, writeAccount } from "../lib/storage";
+import { accountKey, LS } from "../lib/storage";
 import { formatCountOrDash } from "../lib/format";
-import { parseSortState, sortRows, type SortState } from "../lib/columnSort";
+import { sortRows } from "../lib/columnSort";
+import { useSortState } from "../lib/useSortState";
 import { useCardPager } from "../lib/useCardPager";
 import { subsColumn } from "./channelColumns";
 import { notify } from "../lib/notifications";
@@ -44,13 +45,7 @@ export default function ChannelDiscovery({
   // The table/cards density is shared with the Subscriptions tab (ChannelsProvider). The sort is
   // this tab's own (its columns differ), lifted out of the table so the card layout shares it.
   const layout = useChannelLayout();
-  const [sort, setSortState] = useState<SortState>(() =>
-    parseSortState(readAccount(LS.channelDiscoverySort, null)),
-  );
-  const setSort = (s: SortState) => {
-    setSortState(s);
-    writeAccount(LS.channelDiscoverySort, s);
-  };
+  const [sort, setSort] = useSortState(LS.channelDiscoverySort);
 
   const query = useQuery({
     queryKey: qk.discoveredChannels(),