cli: Estimate install ETA from remaining bytes

Flatpak's progress percentage is a heuristic that includes metadata
discovery and write phases. Using it as a time base can produce
misleading remaining times or reach 00:00 while downloads are still
active.

Track the computed transfer total in FlatpakProgress. Use the remaining
bytes and measured transfer rate to calculate the CLI ETA.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f