download: add file size queries and disk space projections
Add common_get_remote_file_size() to query file sizes via HEAD requests
before downloading. Use this in dry-run mode to show:
- Individual file sizes for each item
- Download estimates (total to download, partial, cached)
- Peak space needed during updates (old + new files)
- Projected space during and after download
- Improved low-space warnings based on actual requirements
Also skip loading dynamic backends for the download tool since they
are not needed, and suppress backend initialization output in dry-run
mode for cleaner output.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
