feat: strategy builder

frontend/src/pages/StrategyBuilder.tsx

@@ -57,12 +57,17 @@ function PayoffChart({ priced, spot, scenarioSpot }: { priced: PriceCombo; spot:
     expiry: p.pnl,
     scenario: priced.at_scenario[i]?.pnl,
   }))
+  // Decimals scale with the underlying's own magnitude — an equity at ~740 reads fine
+  // rounded to the unit, but an FX rate at ~1.15 needs several decimals or every tick
+  // collapses to the same rounded label.
+  const decimals = spot < 5 ? 4 : spot < 50 ? 2 : 0
   return (
     <ResponsiveContainer width="100%" height={280}>
       <LineChart data={data} margin={{ top: 8, right: 16, left: 0, bottom: 0 }}>
         <CartesianGrid strokeDasharray="3 3" stroke="#1e2d4d" />
-        <XAxis dataKey="underlying" tick={{ fill: '#475569', fontSize: 10 }} tickLine={false}
-          tickFormatter={(v) => v.toFixed(0)} />
+        <XAxis dataKey="underlying" type="number" domain={['dataMin', 'dataMax']}
+          tick={{ fill: '#475569', fontSize: 10 }} tickLine={false}
+          tickFormatter={(v) => v.toFixed(decimals)} />
         <YAxis tick={{ fill: '#475569', fontSize: 10 }} tickLine={false} axisLine={false}
           tickFormatter={(v) => `${v}`} />
         <Tooltip