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Replace .btn + .btn { margin-left } with margin-right/margin-bottom on
.book-detail-body .btn so the "Im Browser lesen" button stays
left-aligned when the buttons wrap to a new line on narrow viewports.
Also drop the now-unused .reader-nav .btn + .btn override.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>