Add 1-year BTC daily price history chart

New GET /history endpoint fetches 365 days of BTC/EUR data from
CoinGecko, deduplicates by date, and joins the user's purchases.
BTCHistoryChart component renders the price line with orange dot
markers on purchase dates and a dashed cyan avg buy price line.
Tooltip shows purchase details on marked dates.

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

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

@@ -3,6 +3,7 @@ import { useNavigate } from 'react-router-dom';
 import AddPurchase from '../components/AddPurchase';
 import PurchaseList from '../components/PurchaseList';
 import PortfolioChart from '../components/PortfolioChart';
+import BTCHistoryChart from '../components/BTCHistoryChart';
 
 const API = process.env.REACT_APP_API_URL || 'http://localhost:8000';
 
@@ -32,6 +33,7 @@ function StatCard({ label, value, highlight }) {
 export default function Dashboard() {
   const [stats, setStats] = useState(null);
   const [purchases, setPurchases] = useState([]);
+  const [history, setHistory] = useState(null);
   const navigate = useNavigate();
 
   const authHeaders = () => ({
@@ -40,9 +42,10 @@ export default function Dashboard() {
 
   const fetchData = useCallback(async () => {
     try {
-      const [statsRes, purchasesRes] = await Promise.all([
-        fetch(`${API}/stats`, { headers: authHeaders() }),
+      const [statsRes, purchasesRes, historyRes] = await Promise.all([
+        fetch(`${API}/stats`,    { headers: authHeaders() }),
         fetch(`${API}/purchases`, { headers: authHeaders() }),
+        fetch(`${API}/history`,  { headers: authHeaders() }),
       ]);
       if (statsRes.status === 401) {
         localStorage.removeItem('token');
@@ -51,6 +54,7 @@ export default function Dashboard() {
       }
       setStats(await statsRes.json());
       setPurchases(await purchasesRes.json());
+      setHistory(await historyRes.json());
     } catch {
       // silently fail — network may be unavailable
     }
@@ -92,6 +96,7 @@ export default function Dashboard() {
       )}
 
       <PortfolioChart purchases={purchases} stats={stats} />
+      <BTCHistoryChart history={history} stats={stats} />
       <AddPurchase onAdded={fetchData} />
       <PurchaseList purchases={purchases} onChanged={fetchData} />
     </div>