fix(channels): re-hydrate per-account manager state once the account is pinned

The provider tree mounts above <App>, which pins this tab's account only after /api/me
resolves — so on a genuinely fresh tab (empty sessionStorage) the manager's sort, filters,
view and layout initialised from an unknown account and stayed at defaults until a second
reload. Make the active account reactive (a subscribable store + useActiveAccountId via
useSyncExternalStore; setActiveAccount/clearActiveAccount notify) and re-read the stored
value when the account transitions null→id, via a shared useAccountRehydrate hook. A
markDirty guard (called by each owning setter) prevents clobbering a value the user changed
in the brief pre-pin window; the effect is keyed on the account id only, so the user's own
writes can't loop it. useSortState, useFilterState and ChannelsProvider (view/layout) all
adopt it. Affects sort identically — this was a long-standing caveat, not new to R13.

frontend/src/lib/useSortState.ts

@@ -1,5 +1,6 @@
 import { useCallback, useState } from "react";
 import { getAccountRaw, setAccountRaw } from "./storage";
+import { useAccountRehydrate } from "./useAccountRehydrate";
 import { hydrateSort, type SortOption, type SortState } from "./columnSort";
 
 // The per-account persisted sort state, hydrated + healed against the current options — the plumbing
@@ -13,12 +14,17 @@ export function useSortState(
   const [sort, setSort] = useState<SortState>(() =>
     hydrateSort(getAccountRaw(storageBase), options, fallback),
   );
+  // When the account is pinned after mount (first load), re-read the stored sort for that account.
+  const markDirty = useAccountRehydrate(() =>
+    setSort(hydrateSort(getAccountRaw(storageBase), options, fallback)),
+  );
   const set = useCallback(
     (next: SortState) => {
+      markDirty();
       setSort(next);
       setAccountRaw(storageBase, JSON.stringify(next));
     },
-    [storageBase],
+    [storageBase, markDirty],
   );
   return [sort, set];
 }