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/hints.ts

@@ -2,14 +2,10 @@
 // captions on hover so the UI is somewhat self-documenting for new users. Experienced
 // users can turn it off in Settings. Persisted to localStorage (+ server preferences).
 import { createStore } from "./store";
-import { LS } from "./storage";
+import { getAccountRaw, LS, setAccountRaw } from "./storage";
 
 function load(): boolean {
-  try {
-    return localStorage.getItem(LS.hints) !== "0"; // default ON
-  } catch {
-    return true;
-  }
+  return getAccountRaw(LS.hints) !== "0"; // default ON (also when the account isn't known yet)
 }
 
 const store = createStore<boolean>(load());
@@ -19,9 +15,5 @@ export const subscribeHints = store.subscribe;
 
 export function setHintsEnabled(v: boolean): void {
   store.set(v);
-  try {
-    localStorage.setItem(LS.hints, v ? "1" : "0");
-  } catch {
-    /* ignore */
-  }
+  setAccountRaw(LS.hints, v ? "1" : "0");
 }