The nested tbody approach caused browsers to collapse all cells into
one column. Replaced with a single flat x-for loop over flattenedRows
(computed property that interleaves member rows and store sub-rows).
Each row is a single <tr> with 4 proper <td> cells, using x-if to
conditionally render member-level or store-level content per column.
Sub-rows are hidden/shown via expandedMembers array.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>