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/SettingsPanel.tsx

@@ -4,7 +4,7 @@ import { useQueryClient } from "@tanstack/react-query";
 import { Bell, Monitor, Trash2, User } from "lucide-react";
 import { SCHEMES, type Scheme, type ThemePrefs } from "../lib/theme";
 import { api, type Me } from "../lib/api";
-import { LS, usePersistedState } from "../lib/storage";
+import { LS, useAccountPersistedState } from "../lib/storage";
 import Avatar from "./Avatar";
 import { notify, type NotifSettings } from "../lib/notifications";
 import Tooltip from "./Tooltip";
@@ -54,7 +54,7 @@ export default function SettingsPanel({
   onOpenWizard: () => void;
 }) {
   const { t } = useTranslation();
-  const [tabRaw, setTab] = usePersistedState(LS.settingsTab, "appearance");
+  const [tabRaw, setTab] = useAccountPersistedState(LS.settingsTab, "appearance");
   // Clamp at render so a stale/removed tab id falls back to Appearance.
   const tab: TabId = TABS.some((tabItem) => tabItem.id === tabRaw)
     ? (tabRaw as TabId)