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/useSortState.ts

@@ -7,7 +7,7 @@ import { hydrateSort, type SortOption, type SortState } from "./columnSort";
 // key). One home so a new sortable surface (Plex, a future table) gets persistence for free.
 export function useSortState(
   storageBase: string,
-  options: SortOption[],
+  options: SortOption[] = [],
   fallback: SortState = null,
 ): [SortState, (next: SortState) => void] {
   const [sort, setSort] = useState<SortState>(() =>