Move to a custom download domain before wider stable distribution. The current public R2 development URL is rate-limited and intended for development traffic.

Preserve the signed feed, immutable archive URLs, retained old images, and feed cache behavior described in the [desktop update procedure](https://github.com/githubnext/ace2/blob/main/docs/updates.md#hosting-and-publishing).

Tracked by #7. Migrated from [TODO.md — Release](https://github.com/githubnext/ace2/blob/21ac3df5cf6c963cf20e33b0d470ba45eac40150/TODO.md#release).