fix(state): scope all per-account localStorage by account id

Multi-account-in-one-browser (esp. with per-tab accounts) leaked one account's client state
into another via shared localStorage keys. Scope every account-specific key by the tab's active
account (accountKey/readAccount/writeAccount/useAccountPersistedState helpers), so nothing bleeds
across accounts or tabs:

- Real leaks: selected playlist, client notification history + settings, onboarding-dismissed.
- UI position: feed page, channel-manager filter/view + tables, playlist sort, Settings/Stats/
  Users/Config tabs.
- Previously DB-adopted caches (theme, hints, performance mode, sidebar layout, nav/filter
  collapse) — now the cache is per-account too, so there's no flash of the other account's value
  on login.

Kept intentionally global: siftlode.lang (needed pre-login on the Welcome page; the DB pref still
scopes it per-account after sign-in) and siftlode.seenVersion (a per-browser 'new version' banner).
E2EE private keys (IndexedDB) and the chat-dock key were already per-user.

frontend/src/lib/sidebarLayout.ts

@@ -4,7 +4,7 @@
 
 // `show`, `sort` and `content` moved to the feed toolbar (above the cards); they are no
 // longer sidebar widgets. normalizeLayout drops them from any persisted layout automatically.
-import { LS, readJSON, writeJSON } from "./storage";
+import { LS, readAccount, writeAccount } from "./storage";
 
 export type WidgetId = "savedviews" | "date" | "language" | "topic" | "tags";
 
@@ -46,9 +46,9 @@ export function normalizeLayout(raw: unknown): SidebarLayout {
 }
 
 export function loadLayout(): SidebarLayout {
-  return normalizeLayout(readJSON<unknown>(LS.sidebarLayout, {}));
+  return normalizeLayout(readAccount<unknown>(LS.sidebarLayout, {}));
 }
 
 export function saveLayoutLocal(l: SidebarLayout): void {
-  writeJSON(LS.sidebarLayout, l);
+  writeAccount(LS.sidebarLayout, l);
 }