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/frontend/src/pages/Dashboard.js

@@ -28,11 +28,19 @@ const styles = {
   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 }) {
+function StatCard({ label, value, highlight, warning }) {
   return (
     <div style={styles.statCard}>
-      <div style={styles.statLabel}>{label}</div>
+      <div style={{ ...styles.statLabel, display: 'flex', alignItems: 'center', gap: '0.3rem' }}>
+        {label}
+        {warning && (
+          <span title={warning} style={{ color: '#f7931a', cursor: 'default', fontSize: '0.85rem' }}>⚠</span>
+        )}
+      </div>
       <div style={{ ...styles.statValue, ...(highlight ? styles[highlight] : {}) }}>{value}</div>
+      {warning && (
+        <div style={{ color: '#888', fontSize: '0.7rem', marginTop: '0.25rem' }}>{warning}</div>
+      )}
     </div>
   );
 }
@@ -121,7 +129,11 @@ export default function Dashboard() {
           <StatCard label="Total Invested" value={`€${stats.total_invested.toLocaleString()}`} />
           <StatCard label="Avg Buy Price" value={`€${stats.average_price.toLocaleString()}`} />
           <StatCard label="Total BTC" value={`₿${stats.total_btc}`} highlight="neutral" />
-          <StatCard label="Current BTC Price" value={`€${stats.current_price.toLocaleString()}`} />
+          <StatCard
+            label="Current BTC Price"
+            value={`€${stats.current_price.toLocaleString()}`}
+            warning={stats.price_is_cached ? 'Price may be outdated — live fetch failed' : undefined}
+          />
           <StatCard label="Portfolio Value" value={`€${stats.portfolio_value.toLocaleString()}`} />
           <StatCard
             label="Profit / Loss"