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/plexSort.ts

@@ -8,7 +8,7 @@ const MOVIE_SORTS = ["added", "release", "year", "rating", "duration", "title"]
 const SHOW_SORTS = ["added", "release", "year", "rating", "title"] as const;
 
 /** The sort keys a scope offers — movie libraries add `duration`; shows/both don't. */
-export function plexSortKeys(scope: string): string[] {
+function plexSortKeys(scope: string): string[] {
   return [...(scope === "movie" ? MOVIE_SORTS : SHOW_SORTS)];
 }