fix(deflake): AccountsManager useBeta handler off poster-thread + CrossDeviceSyncE2E joins (#1321)

The two deadline-poll/hang flakes that survived #1306's crasher fix:

- AccountsManager.updateAccountSetFromNotification (testUseBetaDidChange 120s hang):
  .TPPUseBetaDidChange is delivered SYNCHRONOUSLY by NotificationCenter on the
  poster's thread (main, from a Settings toggle). The handler did an
  accountSetsLock.sync read (performRead) that blocks until any in-flight
  background catalog-refresh barrier on that lock drains — under CI full-suite
  load that barrier can hold for the whole 120s execution allowance, hanging the
  poster (and, in production, stalling the Settings UI). Dispatch the account-set
  update off the poster's thread (.userInitiated) — it's inherently async (it may
  reload catalogs), so this is behavior-correct AND unblocks the poster.
- CrossDeviceSyncE2ETests (test_audiobookPositionOnDeviceA ~32s): all 10
  wait/fulfillment(timeout:) deadline-polls on fire-and-forget backend annotation
  completions converted to withCheckedContinuation joins. Bookmark types are
  NSObject-based (non-Sendable), so each site projects the asserted primitive
  facts inside the completion (off-isolation) and resumes with a Sendable
  projection — no non-Sendable object crosses the @mainactor boundary.

Verified: 56 tests pass in isolation (51 E2E + 5 AccountsManager).

**Scope:** one critical-path AccountsManager behavior fix (settings-notification
handler no longer blocks the poster) + E2E test-hermeticity. The async dispatch
preserves the account-set update; only the thread it runs on changes.
**Not done:** the broader deadline-poll tail beyond these observed-failing tests.

Co-authored-by: t <t@t.io>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people

Palace/Accounts/Library/AccountsManager.swift

@@ -2002,7 +2002,18 @@ private struct CrawlerHandoffBox: @unchecked Sendable {
     }
 
     @objc private func updateAccountSetFromNotification(_ notif: Notification) {
-        updateAccountSet(completion: nil)
+        // Run off the poster's thread. `.TPPUseBetaDidChange` is delivered
+        // synchronously by `NotificationCenter` on whatever thread posted it —
+        // typically MAIN, from a Settings toggle. `updateAccountSet` does an
+        // `accountSetsLock.sync` read (`performRead`) that blocks until any
+        // in-flight background catalog-refresh barrier on that lock drains; under
+        // load that barrier can take a long time, so reacting synchronously stalls
+        // the poster (the Settings UI — and any test that posts this notification,
+        // which is how it surfaced as a 120s hang). The account-set update is
+        // inherently async anyway (it may reload catalogs), so dispatch it.
+        DispatchQueue.global(qos: .userInitiated).async { [weak self] in
+            self?.updateAccountSet(completion: nil)
+        }
     }
 
     func updateAccountSet(completion: ((Bool) -> Void)?) {