fix: distinguish integrity-check cancellation from failures

[Dingding-leo]

User impact

Stopping the manual library integrity check still returns a safe partial result. An unrelated IndexedDB or service failure merely named AbortError is no longer presented as though the learner pressed Stop.

Scope

• require the audit signal itself to be aborted before classifying an AbortError as user cancellation
• retain cross-realm error-name handling for IndexedDB/fake-indexeddb
• propagate unrelated AbortError failures to the visible error state
• avoid notifying an already unmounted Settings parent when a closing panel finishes abort cleanup
• add a focused regression test for an independently thrown AbortError

Compatibility

No database schema, backup format, scheduler, Service Worker, or Tauri data change.

Release boundary

This draft exists only to apply and locally validate the reviewed source patch. The one-time workflow must remove itself, then the exact final source-only head must pass Web, Chromium, both macOS architectures, final Release checks, and CodeQL before merge.