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/ChannelDiscovery.tsx

@@ -2,6 +2,7 @@ import { useTranslation } from "react-i18next";
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 import { UserPlus } from "lucide-react";
 import { api, HttpError, type DiscoveredChannel } from "../lib/api";
+import { accountKey } from "../lib/storage";
 import { formatViews } from "../lib/format";
 import { notify } from "../lib/notifications";
 import Tooltip from "./Tooltip";
@@ -173,7 +174,7 @@ export default function ChannelDiscovery({
           rows={rows}
           columns={columns}
           rowKey={(c) => c.id}
-          persistKey="siftlode.channelDiscoveryTable"
+          persistKey={accountKey("siftlode.channelDiscoveryTable") ?? undefined}
           controlsPosition="top"
           emptyText={t("channels.discovery.empty")}
         />