refactor(sort): source-agnostic SortControl + SortOption model (R13 S1 foundation)

Introduce the one sort control every content surface will share:
- columnSort.ts gains the SortOption vocabulary + pure pickSort/healSort/
  hydrateSort helpers (direction lives in SortState, never in the key).
- SortControl.tsx: the generic dropdown + asc/desc toggle, with an optional
  "no sort" choice (tables) and a trailing adornment slot (the feed's reshuffle).
- ColumnSortControl is now a thin wrapper over SortControl (API unchanged, so
  Channels + ChannelDiscovery are untouched); optionsFromColumns moved to
  lib/sortOptions.ts to keep the component file fast-refresh clean.
- useSortState.ts: the per-account persist/hydrate/heal plumbing the channel
  manager and discovery each hand-rolled, ready to replace both.

Pure logic is fully unit-tested (columnSort.test.ts 10 -> 23). Gate green:
tsc/eslint/prettier clean, vitest 93 pass. Feed/Plex/channels migration follows.

frontend/src/lib/useSortState.ts

@@ -0,0 +1,24 @@
+import { useCallback, useState } from "react";
+import { getAccountRaw, setAccountRaw } from "./storage";
+import { hydrateSort, type SortOption, type SortState } from "./columnSort";
+
+// The per-account persisted sort state, hydrated + healed against the current options — the plumbing
+// the channel manager and channel discovery each hand-rolled (useState + parseSortState + a persist
+// key). One home so a new sortable surface (Plex, a future table) gets persistence for free.
+export function useSortState(
+  storageBase: string,
+  options: SortOption[],
+  fallback: SortState = null,
+): [SortState, (next: SortState) => void] {
+  const [sort, setSort] = useState<SortState>(() =>
+    hydrateSort(getAccountRaw(storageBase), options, fallback),
+  );
+  const set = useCallback(
+    (next: SortState) => {
+      setSort(next);
+      setAccountRaw(storageBase, JSON.stringify(next));
+    },
+    [storageBase],
+  );
+  return [sort, set];
+}