Add a cancel option when exporting

[TimRl]

frontier-authentication — AbortSignal support for LFS downloads

#997

Summary

Adds cooperative cancellation to downloadLFSFile so callers (e.g. a project
export being cancelled in codex-editor) can abort an in-flight LFS download
near-instantly, without breaking unrelated callers that are sharing the same
deduplicated download.

Changes

• FrontierAPI.downloadLFSFile gains an optional signal?: AbortSignal, threaded
  through getLFSDownloadAction → downloadLFSObject → doDownloadLFSObject and
  into the underlying batch and file fetches.
• Refcounted abort for deduplicated downloads: the shared in-flight download
  is driven by its own AbortController; per-caller signals only stop that
  caller's await (via Promise.race). The shared fetch is aborted only once
  every waiter has aborted — so one caller cancelling can't kill a co-waiter
  downloading the same OID.
• Aborted requests propagate AbortError untouched so callers can distinguish a
  user cancellation from a genuine download failure.
• Hardened the dedup map cleanup (promise.then(clear, clear)) so a rejected
  shared download can't surface as an unhandled rejection or leave a stale entry.

Testing Checklist

• Cancel an export while LFS objects are downloading → downloads abort
  promptly and surface as cancellation, not errors.
• Two callers request the same OID; one aborts → the other still completes.
• All waiters for an OID abort → the shared fetch is actually cancelled (no
  lingering bandwidth use).
• An already-aborted signal never joins/starts a shared download.
• Normal (uncancelled) downloads and the dedup fast-path still work.

[TimRl]

/build

[github-actions]

Pre-release: 0.7.0-pr34-040d442 https://github.com/genesis-ai-dev/frontier-authentication/releases/tag/0.7.0-pr34-040d442