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/theme.ts

@@ -1,4 +1,4 @@
-import { LS, readMerged, writeJSON } from "./storage";
+import { LS, readAccountMerged, writeAccount } from "./storage";
 
 export type Mode = "dark" | "light";
 export type Scheme = "midnight" | "forest" | "slate" | "youtube";
@@ -30,9 +30,9 @@ export function applyTheme(t: ThemePrefs): void {
 }
 
 export function loadLocalTheme(): ThemePrefs {
-  return readMerged(LS.theme, DEFAULT_THEME);
+  return readAccountMerged(LS.theme, DEFAULT_THEME);
 }
 
 export function saveLocalTheme(t: ThemePrefs): void {
-  writeJSON(LS.theme, t);
+  writeAccount(LS.theme, t);
 }