feat: show up/down delta indicators on stats after each refresh

Volume total, transaction count, and top city volumes now display
↑ (green) or ↓ (red) arrows compared to the previous refresh,
making it visible that data is actually updating.

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

src/components/StatsPanel.tsx

@@ -1,3 +1,4 @@
+import { useRef } from 'react';
 import type { PeriodStats } from '../services/DataService';
 
 interface StatsPanelProps {
@@ -9,11 +10,15 @@ interface StatsPanelProps {
 
 const MEDALS = ['🥇', '🥈', '🥉'];
 
-function StatCard({ label, value, sub }: { label: string; value: string; sub?: string }) {
+function StatCard({ label, value, sub, delta }: { label: string; value: string; sub?: string; delta?: 'up' | 'down' | null }) {
   return (
     <div className="bg-[#0f1016] border border-[#2e2f3a] rounded-xl p-4 space-y-1">
       <p className="text-[#4b5563] text-xs uppercase tracking-widest">{label}</p>
-      <p className="text-[#d4a843] text-2xl font-bold tabular-nums">{value}</p>
+      <p className="text-[#d4a843] text-2xl font-bold tabular-nums">
+        {value}
+        {delta === 'up'   && <span className="text-emerald-400 text-sm ml-1.5">↑</span>}
+        {delta === 'down' && <span className="text-red-400    text-sm ml-1.5">↓</span>}
+      </p>
       {sub && <p className="text-[#6b7280] text-xs">{sub}</p>}
     </div>
   );
@@ -21,6 +26,26 @@ function StatCard({ label, value, sub }: { label: string; value: string; sub?: s
 
 export function StatsPanel({ stats, loading, periodDays, source }: StatsPanelProps) {
   const periodLabel = periodDays === 1 ? '24 dernières heures' : `${periodDays} derniers jours`;
+  const prevStats = useRef<PeriodStats | null>(null);
+
+  // Calcule le delta d'une valeur par rapport au refresh précédent
+  function delta(current: number, prevMap: Map<string, number>, key: string) {
+    const prev = prevMap.get(key);
+    if (prev === undefined) return null;
+    if (current > prev) return <span className="text-emerald-400 text-xs ml-1">↑</span>;
+    if (current < prev) return <span className="text-red-400 text-xs ml-1">↓</span>;
+    return null;
+  }
+
+  // Construit une map volume précédent par ville
+  const prevCityVolume = new Map(
+    (prevStats.current?.topCities ?? []).map((c) => [c.name, c.volume])
+  );
+  const prevVolume   = prevStats.current?.totalVolume ?? null;
+  const prevTxCount  = prevStats.current?.transactionCount ?? null;
+
+  // Mémorise les stats après le rendu
+  if (stats && !loading) prevStats.current = stats;
   const geoPct = stats && stats.transactionCount > 0
     ? Math.round((stats.geoCount / stats.transactionCount) * 100)
     : null;
@@ -55,11 +80,13 @@ export function StatsPanel({ stats, loading, periodDays, source }: StatsPanelPro
           <StatCard
             label="Volume total"
             value={`${stats.totalVolume.toLocaleString('fr-FR', { maximumFractionDigits: 2 })} Ğ1`}
+            delta={prevVolume !== null ? (stats.totalVolume > prevVolume ? 'up' : stats.totalVolume < prevVolume ? 'down' : null) : null}
           />
           <StatCard
             label="Transactions"
             value={stats.transactionCount.toLocaleString('fr-FR')}
             sub={`≈ ${(stats.totalVolume / (stats.transactionCount || 1)).toFixed(2)} Ğ1 / tx`}
+            delta={prevTxCount !== null ? (stats.transactionCount > prevTxCount ? 'up' : stats.transactionCount < prevTxCount ? 'down' : null) : null}
           />
           {/* Couverture géo — uniquement en mode live */}
           {source === 'live' && geoPct !== null && (
@@ -96,8 +123,9 @@ export function StatsPanel({ stats, loading, periodDays, source }: StatsPanelPro
                 <p className="text-white text-sm font-medium truncate">{city.name}</p>
                 <p className="text-[#6b7280] text-xs">{city.count} tx</p>
               </div>
-              <span className="text-[#d4a843] text-sm font-mono shrink-0">
+              <span className="text-[#d4a843] text-sm font-mono shrink-0 flex items-center gap-1">
                 {city.volume.toLocaleString('fr-FR', { maximumFractionDigits: 0 })} Ğ1
+                {delta(city.volume, prevCityVolume, city.name)}
               </span>
             </div>
           ))}