On first open the ListView started at offset 0 (future end), leaving
today's pill out of view. Also, navigation always scrolled the pill to
the left edge instead of centering it.
Added _centeredOffset() helper and a post-frame jumpTo in initState()
so the initially selected date is centered immediately. Updated
_jumpToToday() and didUpdateWidget() to use the same centering formula.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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