Add purchase date picker and sells feature

- Purchase form now includes a date picker (defaults to today)
- New Sell model, CRUD endpoints (/sells), and stats integration
- AddSell and SellList components added to dashboard
- Portfolio chart updated to reflect sells over time

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

btc-portfolio/frontend/src/components/PortfolioChart.js

@@ -42,12 +42,13 @@ function priceOn(date, priceMap, currentPrice, isToday, sortedPurchases) {
   return fallback;
 }
 
-export default function PortfolioChart({ purchases, stats, btcHistory }) {
+export default function PortfolioChart({ purchases, sells, stats, btcHistory }) {
   const chartRef = useRef(null);
 
   if (!purchases || purchases.length === 0) return null;
 
   const sorted = [...purchases].sort((a, b) => new Date(a.created_at) - new Date(b.created_at));
+  const sortedSells = [...(sells || [])].sort((a, b) => new Date(a.created_at) - new Date(b.created_at));
   const today = new Date();
   today.setHours(0, 0, 0, 0);
 
@@ -89,6 +90,14 @@ export default function PortfolioChart({ purchases, stats, btcHistory }) {
         cumInvested += p.amount_eur;
       }
     });
+    sortedSells.forEach(s => {
+      const sDate = new Date(s.created_at);
+      sDate.setHours(0, 0, 0, 0);
+      if (sDate <= date) {
+        cumBtc -= s.btc_amount;
+        cumInvested -= s.btc_amount * s.price_eur;
+      }
+    });
 
     if (cumBtc === 0) return; // no purchases yet at this date