The single-row header packed nav + 비교/공유/Star + 7-tab PeriodTabs side by
side. On narrow widths the line wrapped with PeriodTabs landing mid-row, which
looked broken. Move PeriodTabs to its own row underneath, right-aligned with
overflow-x-auto fallback. Action group keeps flex-wrap as a defensive fallback.
Pure layout — no behavior change.
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