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.dom.test.ts

@@ -12,6 +12,9 @@ vi.mock("./storage", () => ({
   setAccountRaw: (k: string, v: string) => {
     store.set(k, v);
   },
+  // A stable account id keeps useAccountRehydrate a no-op here (no null→id transition); the re-hydrate
+  // path itself is covered in useAccountRehydrate.dom.test.ts.
+  useActiveAccountId: () => 1,
 }));
 
 import { useSortState } from "./useSortState";