Add chart view toggle: Both / Portfolio / 1-Year BTC

Tab bar above charts lets the user switch between showing both charts
stacked, or either chart individually.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

btc-portfolio/frontend/src/pages/Dashboard.js

@@ -19,6 +19,9 @@ const styles = {
   positive: { color: '#6bff8e' },
   negative: { color: '#ff6b6b' },
   neutral: { color: '#f7931a' },
+  tabs: { display: 'flex', gap: '0.5rem', marginBottom: '1rem' },
+  tab: { padding: '0.45rem 1.1rem', borderRadius: '8px', border: '1px solid #444', background: 'none', color: '#888', cursor: 'pointer', fontSize: '0.9rem' },
+  tabActive: { padding: '0.45rem 1.1rem', borderRadius: '8px', border: '1px solid #f7931a', background: 'rgba(247,147,26,0.1)', color: '#f7931a', cursor: 'pointer', fontSize: '0.9rem', fontWeight: 700 },
 };
 
 function StatCard({ label, value, highlight }) {
@@ -34,6 +37,7 @@ export default function Dashboard() {
   const [stats, setStats] = useState(null);
   const [purchases, setPurchases] = useState([]);
   const [history, setHistory] = useState(null);
+  const [chartView, setChartView] = useState('both'); // 'both' | 'portfolio' | 'history'
   const navigate = useNavigate();
 
   const authHeaders = () => ({
@@ -95,8 +99,17 @@ export default function Dashboard() {
         </div>
       )}
 
-      <PortfolioChart purchases={purchases} stats={stats} />
-      <BTCHistoryChart history={history} stats={stats} />
+      <div style={styles.tabs}>
+        {[['both', 'Both'], ['portfolio', 'Portfolio'], ['history', '1-Year BTC']].map(([key, label]) => (
+          <button
+            key={key}
+            style={chartView === key ? styles.tabActive : styles.tab}
+            onClick={() => setChartView(key)}
+          >{label}</button>
+        ))}
+      </div>
+      {(chartView === 'both' || chartView === 'portfolio') && <PortfolioChart purchases={purchases} stats={stats} />}
+      {(chartView === 'both' || chartView === 'history')   && <BTCHistoryChart history={history} stats={stats} />}
       <AddPurchase onAdded={fetchData} />
       <PurchaseList purchases={purchases} onChanged={fetchData} />
     </div>