The active period tab previously used `bg-zinc-700`, which blends with the
zinc-toned header buttons (비교 / 공유 / Star) right beside it — users had to
read the tab label to know which period was loaded.
Switch active style to rose-500 (한국 상승 컬러 톤 — same family as positive
returns and target lines elsewhere in the app), with bold weight and a soft
glow. Inactive tabs unchanged. Also adds tablist/tab/aria-selected semantics
so screen readers announce the active period.
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