Converts the single-column stacked filters popover into a 2-column grid
layout. Status filters appear in the left column, Priority + Assignee in
the right column. On mobile (<640px), falls back to single column.
Popover width increased from 320px to 480px with viewport-aware max to
prevent overflow.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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