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.test.ts

@@ -169,4 +169,14 @@ describe("hydrateSort", () => {
       dir: "desc",
     });
   });
+
+  it("parses without healing when no options are supplied (caller heals for display)", () => {
+    // A table/provider that owns no vocabulary keeps whatever was stored, even an unknown key.
+    expect(hydrateSort(JSON.stringify({ key: "anything", dir: "desc" }), [])).toEqual({
+      key: "anything",
+      dir: "desc",
+    });
+    // A malformed stored value still coerces to the fallback.
+    expect(hydrateSort(JSON.stringify({ key: 3, dir: "up" }), [], null)).toBeNull();
+  });
 });