fix(datasets): ship and preflight downloaders used by automatic recipes

[HaoxuanLiTHUAI]

Summary

The dataset marketplace advertises several entries as automatic public downloads, but the main runtime image does not ship the command-line tools used by their recipes.

Reproduction

On main@3732f9e3, start downloads from the dataset marketplace.

Observed on the 208 preview

• OpenNeuro fails with: Required downloader 'datalad' is not installed or not on PATH.
• The main container also lacks dandi and wget.
• BCI Competition IV 2a succeeds because its recipe uses the built-in HTTP downloader.
• A completed BCI job still presents “下载数据 / 开始下载” instead of a completed/open-location action.

Expected

Every recipe advertised as an automatic download must be executable in the published main image, or the UI must preflight and disable it with actionable setup guidance before starting a doomed job.

Acceptance criteria

• Published main images support the datalad, dandi, and wget recipes, with build-time smoke assertions.
• Downloader readiness is covered by tests.
• Completed jobs do not present the same primary action as a never-started job.
• Retry remains available for failed jobs.