fix: LFO→cutoff modulation, visual knob feedback, persistent hints

- Fix LFO→Filter cutoff: add scaling Gain nodes so LFO (-1..1) maps to
  meaningful Hz modulation (±cutoff value). Same fix for LFO→Osc freq.
  Mod scale updates dynamically when user changes the base param value.
- Visual modulation indicator: knobs receiving LFO/modulation show a
  pulsing dashed ring animation (spin + pulse) around the knob arc
- Persist hint usage per level: using a hint permanently caps that level
  at 2 stars — survives reload/restart. No more cheating by restarting!
- Hint state stored in separate localStorage key (synthquest-hints)
- Admin reset also clears hint history

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

src/components/Knob.jsx

@@ -19,7 +19,7 @@ function describeArc(cx, cy, r, startDeg, endDeg) {
   return `M ${start.x} ${start.y} A ${r} ${r} 0 ${large} 0 ${end.x} ${end.y}`;
 }
 
-export default function Knob({ value, min, max, onChange, color = 'var(--accent)', label, unit, formatValue }) {
+export default function Knob({ value, min, max, onChange, color = 'var(--accent)', label, unit, formatValue, modulated = false }) {
   const ref = useRef(null);
   const dragRef = useRef(null);
   const inputRef = useRef(null);
@@ -122,9 +122,13 @@ export default function Knob({ value, min, max, onChange, color = 'var(--accent)
   }
 
   return (
-    <div className="knob-container" onWheel={handleWheel} onDoubleClick={handleDoubleClick}>
+    <div className={`knob-container ${modulated ? 'knob-modulated' : ''}`} onWheel={handleWheel} onDoubleClick={handleDoubleClick}>
       <svg className="knob-svg" viewBox={`0 0 ${SIZE} ${SIZE}`}
         onPointerDown={handlePointerDown} ref={ref}>
+        {/* Modulation glow ring */}
+        {modulated && (
+          <circle className="knob-mod-ring" cx={cx} cy={cy} r={RADIUS + 1} style={{ stroke: color }} />
+        )}
         <path className="knob-track" d={trackPath} />
         {fillPath && <path className="knob-fill" d={fillPath} style={{ stroke: color }} />}
         <circle className="knob-dot" cx={dotPos.x} cy={dotPos.y} r={2} />