feat: download shared links

[Findddx]

Summary

• add anonymous and organization-scoped shared-link browsing and downloads
• add lazy single-level terminal picker with multi-select and safe nested local writes
• add non-interactive JSON listing/download flows and capability redaction
• rewrite user documentation and prepare v0.3.0 packages

Verification

• make test
• CGO_ENABLED=0 go test ./...
• focused race tests repeated for sharelink, picker, download, CLI, and integration
• live anonymous public-link list/download validation without account credentials or remote writes
• two reproducible package builds compared byte-for-byte