Cancelled replies, edit-regenerate, and provider errors now surface clearly

Three chat reliability fixes.

Cancelling a reply mid-stream no longer leaves a spinner stuck on “Generating…” until reload. The bubble now resolves to “Request was interrupted”, persists across reload, and offers a Retry action that resubmits the original message.

Selecting the Auto model and using Edit or Save & Regenerate previously returned a provider error because the model resolver did not run on the regenerate path. The resolver is now applied to all regenerate entry points; Auto behaves identically across new messages, edits, and regenerations.

Slow or failing upstream providers previously left the assistant bubble frozen with no indication of why. The bubble now resolves to a clear message identifying whether the request timed out, was rate-limited, or hit a provider error, and offers Retry.

No action required.