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/storage.ts

@@ -1,4 +1,4 @@
-import { useCallback, useState } from "react";
+import { useCallback, useState, useSyncExternalStore } from "react";
 
 // Central registry of every localStorage key the app uses. One place to see them all, rename
 // safely, and avoid collisions — instead of `"siftlode.x"` string literals scattered across
@@ -83,6 +83,25 @@ export function activeAccountId(): number | null {
   }
 }
 
+// Reactive active-account. setActiveAccount/clearActiveAccount write sessionStorage — a silent op that
+// triggers no React render — so this tab's per-account state (sort/filter/view/layout, all keyed by the
+// account) would never re-read once the account is finally known. That bites exactly once, on first
+// load: the provider tree above <App> mounts with NO account, then <App> pins it after /api/me resolves.
+// The account setters call notifyActiveAccountChange() so useActiveAccountId subscribers can re-hydrate.
+const accountListeners = new Set<() => void>();
+function subscribeActiveAccount(cb: () => void): () => void {
+  accountListeners.add(cb);
+  return () => accountListeners.delete(cb);
+}
+export function notifyActiveAccountChange(): void {
+  for (const cb of [...accountListeners]) cb();
+}
+/** The current tab's active account id, as reactive state — re-renders when it is (re)pinned. The
+ * snapshot is a primitive (number | null), so useSyncExternalStore's Object.is check is stable. */
+export function useActiveAccountId(): number | null {
+  return useSyncExternalStore(subscribeActiveAccount, activeAccountId, activeAccountId);
+}
+
 /** Suffix a base localStorage key with an account id so per-account UI state (filters, selected
  * playlist, notifications, tab positions, cached prefs…) is NOT shared across the accounts signed
  * into one browser — a bare shared key leaks one account's state into another (and, with per-tab