Fix progress bar remaining time calculation

[lprnmns]

Problem

The custom ProgressBar remaining-time placeholder always reports less than one second after progress begins. For a bar with 10 total steps, 2 completed steps, and 10 seconds elapsed, it reports "< 1 sec" even though 40 seconds remain.

Fix

Use the number of uncompleted steps when estimating the remaining time. This makes remaining equal to estimated completion time minus elapsed time.

Tests

• poetry run pytest tests/ui/test_progress_bar.py -k remaining - passed (1 passed, 25 deselected)
• poetry run pytest - passed (248 passed, 3 skipped)
• poetry run pre-commit run --all-files - passed
• poetry run mypy - passed
• towncrier check --compare-with origin/main - passed

Compatibility

This changes only the documented custom remaining-time calculation. The normal and verbose built-in formats do not use the remaining placeholder.

Related issue

Independent reproduction; no matching open issue or pull request was found.

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