Lift shared state (displayColumnNames, flowIndex) from per-row/per-cell
useAppSelector calls to the FlowTable parent component. This eliminates
N identical Redux subscriptions across all visible rows.

Changes:
- FlowTable.tsx: Fetch displayColumnNames from state.options.web_columns
  in mapStateToProps; pass as prop to FlowRow along with flowIndex.
- FlowRow.tsx: Accept displayColumnNames and flowIndex as props instead
  of useAppSelector. Remove unused useAppSelector import.
- FlowColumns.tsx: Accept optional flowIndex prop in index column;
  use it instead of per-cell useAppSelector(_listIndex).
- FlowTableHead.tsx: Add null guard on displayColumnNames for
  edge-case during initial state hydration.
- FlowRowSpec.tsx: Update test to pass new required props.

The index column now uses flowIndex prop instead of
useAppSelector. Pass flowIndex={0} in the generic column
render loop so the snapshot matches the expected numeric
output.