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/onboarding.ts

@@ -12,9 +12,11 @@
 //                                       suppresses the auto-popup on future logins (they
 //                                       can still reopen it from Settings → Account).
 
-import { LS } from "./storage";
+import { getAccountRaw, LS, setAccountRaw } from "./storage";
 
 export const ONBOARD_ACTIVE = "siftlode.onboarding.active"; // sessionStorage (not in LS, which is localStorage)
+// Per-account (see accountKey): a fresh account should still get the onboarding nudge even if a
+// different account in the same browser dismissed it.
 export const ONBOARD_DISMISSED = LS.onboardingDismissed;
 
 export function shouldAutoOpenOnboarding(me: {
@@ -28,7 +30,7 @@ export function shouldAutoOpenOnboarding(me: {
   // email+password only — then there's nothing to connect, so don't nudge.
   if (me.google_enabled === false) return false;
   if (sessionStorage.getItem(ONBOARD_ACTIVE)) return true;
-  return !me.can_read && !localStorage.getItem(ONBOARD_DISMISSED);
+  return !me.can_read && !getAccountRaw(ONBOARD_DISMISSED);
 }
 
 /** Mark a grant as in progress, then send the user to Google's consent screen. */
@@ -40,5 +42,5 @@ export function beginGrant(access: "read" | "write"): void {
 /** Leave the wizard. `dismiss` suppresses the future auto-popup (used when read is skipped). */
 export function endOnboarding(dismiss: boolean): void {
   sessionStorage.removeItem(ONBOARD_ACTIVE);
-  if (dismiss) localStorage.setItem(ONBOARD_DISMISSED, "1");
+  if (dismiss) setAccountRaw(ONBOARD_DISMISSED, "1");
 }