fix(desktop-gpui): align upload chunks for Google Drive

[yashranaway]

google drive resumable uploads need every chunk except the last to be a multiple of 256 KiB. the chunk size was just the file size over 100, clamped to 5 to 15 MiB, so most exports got odd sizes, a 601 MiB export used 6,301,941 byte chunks, and drive rejects those. retrying the same chunk can't help.

now the size gets rounded down to 256 KiB inside the same bounds. sizing is shared, so drive urls that come back without provider info are covered too, and s3 parts just get slightly rounder sizes. the drive loopback test now goes through the real sizing instead of a fixed 1000 bytes.

Confidence Score: 5/5

This PR appears safe to merge.

Summary

Rounds upload chunks down to multiples of 256 KiB so Google Drive accepts intermediate chunks.

• Multipart uploads send aligned full chunks to Google Drive.

_{Reviews (1) · Last reviewed commit: "fix(desktop-gpui): align upload chunks f..." · Reviewed by Greptile}

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[yashranaway]

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