Add partial range download and head/tail preview commands

[goanpeca]

Summary

Opening a large object currently implies downloading the whole object. The SDK supports range requests, so the UI can expose focused preview commands for the first bytes, last bytes, or a custom byte range.

SDK capability

• Bucket.download(fileName, { range, ... })
• B2Object.downloadById(fileId, { range, ... })
• B2Object.head(...) / B2Object.headById(...) for size and metadata

Proposed UI

Add context-menu commands such as Preview Head, Preview Tail, and Download Byte Range for files. This gives users a safe way to inspect logs, archives, and very large objects without downloading everything.

Acceptance criteria

• Users can preview the first N bytes of a file.
• Users can preview the last N bytes of a file when size metadata is available.
• Users can request a custom byte range and save or open the result.
• The UI validates ranges before making the SDK call.
• Tests cover range construction, invalid ranges, and command visibility.

Related

• Download files to a chosen location (Save As / download to folder) #57 tracks general file download support.
• Show file version history (list, open, restore, delete a version) #59 tracks version-specific open/restore/delete flows.