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/components/Tabs.tsx

@@ -1,4 +1,4 @@
-import { usePersistedState } from "../lib/storage";
+import { useAccountPersistedState } from "../lib/storage";
 
 // Reusable horizontal pill tab-rail for in-page sub-navigation (Configuration, Users & roles…).
 // The active tab is persisted to localStorage so a reload (F5) keeps the user where they were,
@@ -10,9 +10,9 @@ export interface TabDef {
   badge?: number; // optional count pill (e.g. pending requests); hidden when 0/undefined
 }
 
-/** Persisted active-tab state — the canonical helper now lives in lib/storage as
- * `usePersistedState`; re-exported here under its original name for existing call sites. */
-export const usePersistedTab = usePersistedState;
+/** Persisted active-tab state, scoped to the current account (so one account's last tab doesn't
+ * carry into another in the same browser) — re-exported under its original name for call sites. */
+export const usePersistedTab = useAccountPersistedState;
 
 export default function Tabs({
   tabs,