Guard against division by zero in progress speed calculation

[Copilot]

Addresses potential division by zero when calculating transfer speed in progress tracking callbacks. When operations complete in <1ms, elapsedSeconds can be 0, resulting in Infinity for speedMBps.

Changes:

• Added guard in zip.ts and upload.ts to return 0 when elapsedSeconds < 0.001
• Prevents "Infinity MB/s" from appearing in progress output during initial callback invocations

// Before
speedMBps: written / elapsedSeconds / 1024 / 1024

// After  
const speedMBps = elapsedSeconds < 0.001 ? 0 : written / elapsedSeconds / 1024 / 1024

Speed displays as "0.00 MB/s" initially, then updates to actual rate as data transfers.

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