Cache last known BTC price and show stale warning in UI

When the CoinGecko API fails, fall back to the last successful price
instead of 0.0, and surface a warning indicator on the price card.

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

btc-portfolio/backend/app/routes/stats.py

@@ -26,7 +26,7 @@ def get_stats(
     net_btc = total_btc_bought - total_btc_sold
     net_invested = total_invested - proceeds_eur
     average_price = net_invested / net_btc if net_btc > 0 else 0.0
-    current_price = get_btc_price_eur()
+    current_price, price_is_cached = get_btc_price_eur()
     portfolio_value = net_btc * current_price
     profit_loss = portfolio_value - net_invested
 
@@ -35,6 +35,7 @@ def get_stats(
         "total_btc": round(net_btc, 8),
         "average_price": round(average_price, 2),
         "current_price": round(current_price, 2),
+        "price_is_cached": price_is_cached,
         "portfolio_value": round(portfolio_value, 2),
         "profit_loss": round(profit_loss, 2),
     }