Advertise Windows and Linux AppImage downloads

The download page now advertises Windows and Linux, with platform-specific S3 paths and checksums. Linux downloads a single MappleStrudel.AppImage with clear launch instructions; Windows continues to download the playable ZIP. Switching platforms cancels stale checksum requests so a Windows hash cannot be shown beside a Linux package.

The client release publishes the matching platform objects before this website change reaches production. Five focused React tests passed on the current production base; the production frontend build and both platform controls were also checked locally. Existing unrelated lint warnings remain documented in DOWNLOADS.md.