fix: add position tick on colorize sliders so cursor is visible at baseline

At baseline (deviation=0) the colored bar had width=0 and disappeared.
Added a 2px vertical tick mark at the current slider position so the
handle location is always visible even when there's no deviation.

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

frontend/src/pages/EuroSimulator.tsx

@@ -163,6 +163,7 @@ function Slider({
         )}>{format(value)}</span>
       </div>
       <div className="relative h-1.5 bg-dark-900 rounded overflow-hidden">
+        {/* Deviation bar (colored, from center to current value) */}
         <div
           className={clsx('absolute h-full rounded transition-all', trackColor)}
           style={colorize
@@ -170,6 +171,13 @@ function Slider({
             : { left: 0, width: `${pct}%` }
           }
         />
+        {/* Position tick — always visible so cursor location is clear at baseline */}
+        {colorize && (
+          <div
+            className="absolute top-0 bottom-0 w-0.5 bg-slate-400 transition-all"
+            style={{ left: `${Math.min(98.5, pct)}%` }}
+          />
+        )}
         <input
           type="range" min={min} max={max} step={step} value={value}
           onChange={e => onChange(parseFloat(e.target.value))}