### Context

`OpenProject::Common::BorderBoxListComponent` accepts a `header_padding:` option (`:inherit`, `:condensed`, `:default`, `:spacious`) that overrides the Begin of the insertionheader'sEnd of the insertion Begin of the deletion header'sEnd of the deletion vertical padding independently of the Begin of the insertionbox'sEnd of the insertion Begin of the deletion box'sEnd of the deletion own `padding:`. It says a second time what `padding:` already says, and each tier quietly carries a second effect: its own value for `--op-border-box-list-header-row-gap`, the gap below the header title line.

That coupling is what produced #AGILE-397. Moving the backlogs lists to `header_padding: :condensed` zeroed the gap in a header nobody was thinking about, and the loss only surfaced in the sprint headers of the Agile Backlogs page.

### Goal

Remove `header_padding:` so a Begin of the insertionlist'sEnd of the insertion Begin of the deletion list'sEnd of the deletion padding is expressed once, through `padding:`.

### Notes

The only callers are the two backlogs components (inbox and work package card list). Beyond those, the change touches the component, its spec, the Lookbook previews and the Lookbook docs page.

Removing the option changes the rendered header `padding-block` for those callers, so this needs a visual pass over the backlogs pages rather than a spec-only check.

Begin of the insertionSuggestedEnd of the insertion Begin of the deletion Agreed between Alexander and HenrietteEnd of the deletion while fixing #AGILE-397: https://github.com/opf/openproject/pull/24865