feat(frontend): add HistoryCard + HistoryList

frontend/src/components/HistoryCard.astro

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+---
+import { HistoryList } from './HistoryList';
+interface Props { station: string; }
+const { station } = Astro.props;
+---
+<div class="card history">
+  <div class="full-secthead">
+    <div class="full-secthead-jp">再生履歴</div>
+    <div class="full-secthead-en">// recently played</div>
+  </div>
+  <HistoryList station={station} client:visible />
+</div>

frontend/src/components/HistoryList.tsx

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+import { useEffect, useState } from 'react';
+import type { HistoryEntry } from '@lib/types';
+import { fmtRelative } from '@lib/format';
+
+interface Props {
+  station: string;
+}
+
+export function HistoryList({ station }: Props) {
+  const [entries, setEntries] = useState<HistoryEntry[] | null>(null);
+  const [now, setNow] = useState(() => new Date());
+
+  useEffect(() => {
+    const ctrl = new AbortController();
+    const load = async () => {
+      try {
+        const r = await fetch(`/now-playing/${station}.history.json`, { signal: ctrl.signal });
+        if (!r.ok) { setEntries([]); return; }
+        const arr = await r.json() as HistoryEntry[];
+        setEntries(Array.isArray(arr) ? arr : []);
+      } catch (err) {
+        if ((err as Error).name !== 'AbortError') setEntries([]);
+      }
+    };
+    void load();
+    const id = setInterval(load, 15_000);
+    const tick = setInterval(() => setNow(new Date()), 30_000);
+    return () => { ctrl.abort(); clearInterval(id); clearInterval(tick); };
+  }, [station]);
+
+  if (entries === null) return <div className="history-empty">loading…</div>;
+  if (!entries.length) return <div className="history-empty">no recent tracks</div>;
+
+  return (
+    <div className="history-list">
+      {entries.slice(0, 10).map((e, i) => {
+        const start = new Date(parseInt(e.started_at, 10) * 1000);
+        return (
+          <div key={`${e.started_at}-${i}`} className="history-row">
+            <span className="t">{fmtRelative(start, now)}</span>
+            <span className="title">{e.title || '—'}</span>
+            <span className="artist">{e.artist || '—'}</span>
+          </div>
+        );
+      })}
+    </div>
+  );
+}

frontend/src/styles/components/history-card.css

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+.full-secthead { margin-bottom: 14px; }
+.full-secthead-jp { font-family: var(--f-pixel); font-size: 18px; color: var(--lemon); letter-spacing: 4px; }
+.full-secthead-en { font-family: var(--f-pixel); font-size: 22px; color: var(--cream); letter-spacing: 1px; margin-top: 2px; }
+
+.card {
+  background: var(--plum);
+  border: 2px solid var(--ink);
+  padding: 22px 24px;
+  position: relative;
+}
+.card::before {
+  content: ""; position: absolute; inset: 4px; border: 1px dashed #ff3ea522; pointer-events: none;
+}
+
+.history-list { display: flex; flex-direction: column; }
+.history-row {
+  display: grid; grid-template-columns: 50px 1fr auto;
+  gap: 12px; padding: 8px 4px;
+  border-bottom: 1px dashed #ff3ea522;
+  font-size: 13px;
+}
+.history-row:last-child { border-bottom: none; }
+.history-row .t { font-family: var(--f-mono); color: var(--cyan); }
+.history-row .title { font-family: var(--f-pixel); font-size: 14px; color: var(--cream); }
+.history-row .artist { font-family: var(--f-mono); color: #fff4e8aa; font-size: 12px; }
+.history-empty {
+  font-family: var(--f-mono); font-size: 13px; color: #fff4e8aa; padding: 8px 4px;
+}