fix: VIX slider bar overflow — clamp pivot to range, add center prop

Slider was computing mid = (0 - 10) / (60 - 10) * 100 = -20% for VIX
(0 outside the 10-60 range), causing the colored track to start at a negative
left position and overflow the container.

Fix: add overflow-hidden on the track div; add center prop (defaults 0)
that the caller sets to the actual baseline value when 0 is out of range.
VIX now pivots at 18, real yield US at 2.1.

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

frontend/src/pages/EuroSimulator.tsx

@@ -130,21 +130,25 @@ function compute(p: Params): ModelResult {
 
 function Slider({
   label, value, min, max, step, format, onChange,
-  colorize = false, reverse = false,
+  colorize = false, reverse = false, center,
 }: {
   label: string; value: number; min: number; max: number; step: number
   format: (v: number) => string
   onChange: (v: number) => void
   colorize?: boolean
   reverse?: boolean  // true = value goes up → negative (red)
+  center?: number   // neutral pivot point (defaults to 0; use baseline value if 0 is out of range)
 }) {
   const pct = ((value - min) / (max - min)) * 100
-  const mid = ((0 - min) / (max - min)) * 100
+  const pivot = center ?? 0
+  // Clamp mid to [0,100] so the track never overflows the container
+  const mid = Math.max(0, Math.min(100, ((pivot - min) / (max - min)) * 100))
 
+  const atCenter = Math.abs(value - pivot) < step / 2
   let trackColor = 'bg-blue-500'
   if (colorize) {
-    const isPositive = reverse ? value < 0 : value > 0
+    const isPositive = reverse ? value < pivot : value > pivot
-    trackColor = value === 0 ? 'bg-slate-600' : isPositive ? 'bg-emerald-500' : 'bg-rose-500'
+    trackColor = atCenter ? 'bg-slate-600' : isPositive ? 'bg-emerald-500' : 'bg-rose-500'
   }
 
   return (
@@ -153,12 +157,12 @@ function Slider({
         <span className="text-xs text-slate-400">{label}</span>
         <span className={clsx(
           'text-xs font-mono font-semibold',
-          colorize && value !== 0
+          colorize && !atCenter
-            ? (reverse ? value < 0 : value > 0) ? 'text-emerald-400' : 'text-rose-400'
+            ? (reverse ? value < pivot : value > pivot) ? 'text-emerald-400' : 'text-rose-400'
             : 'text-white',
         )}>{format(value)}</span>
       </div>
-      <div className="relative h-1.5 bg-dark-900 rounded">
+      <div className="relative h-1.5 bg-dark-900 rounded overflow-hidden">
         <div
           className={clsx('absolute h-full rounded transition-all', trackColor)}
           style={colorize
@@ -407,12 +411,12 @@ export default function EuroSimulator() {
           <Section title="📊 Marchés & sentiment" color="border-slate-700/30 bg-dark-800/60">
             <Slider label="VIX" value={p.vix} min={10} max={60} step={0.5}
               format={v => v.toFixed(1)} onChange={v => set('vix', v)}
-              colorize reverse />
+              colorize reverse center={BASE.vix} />
             <Slider label="Pétrole (USD/bbl)" value={p.oil} min={40} max={130} step={1}
               format={v => `$${v.toFixed(0)}`} onChange={v => set('oil', v)} />
             <Slider label="Taux réel US 10Y" value={p.real_yield_us} min={-1} max={4} step={0.05}
               format={v => `${v > 0 ? '+' : ''}${v.toFixed(2)}%`}
-              onChange={v => set('real_yield_us', v)} colorize reverse />
+              onChange={v => set('real_yield_us', v)} colorize reverse center={BASE.real_yield_us} />
           </Section>
         </div>