fix: replace Inter with Space Grotesk (#audit-P3)

Inter was flagged as the only AI tell in the audit. Space Grotesk
fits the pedal brand — mechanical, precise character with excellent
small-screen readability.

Changes:
- src/index.css: body font-family Inter → Space Grotesk
- src/App.jsx: Google Fonts import + body font-family + inline style
- src/GlobalSettings.jsx, GlobalEQ.jsx, ModelBrowser.jsx, FocusView.jsx:
  inline fontFamily Inter → Space Grotesk
- index.html: added Google Fonts preconnect + Space Grotesk import
- docs/helix-stadium-mockup.html, docs/helix-stadium-concept.html: updated
  font imports and font-family

src/GlobalSettings.jsx

@@ -394,7 +394,7 @@ export default function GlobalSettings({ onClose, onMasterVolume, onAudioProfile
   }, [savedValues]);
 
   return (
-    <div style={{ position: "fixed", inset: 0, zIndex: 998, display: "flex", flexDirection: "column", background: T.bg, color: T.textPrimary, fontFamily: "'Inter', sans-serif" }}>
+    <div style={{ position: "fixed", inset: 0, zIndex: 998, display: "flex", flexDirection: "column", background: T.bg, color: T.textPrimary, fontFamily: "'Space Grotesk', sans-serif" }}>
       <div style={{ display: "flex", alignItems: "center", justifyContent: "space-between", padding: "10px 14px", borderBottom: `1px solid ${T.border}`, background: T.panel, flexShrink: 0 }}>
         <div style={{ display: "flex", alignItems: "center", gap: 10 }}>
           <button onClick={onClose} style={{ width: 34, height: 34, borderRadius: 8, display: "flex", alignItems: "center", justifyContent: "center", background: T.surface, border: `1px solid ${T.border}`, color: T.textPrimary, fontSize: 16, cursor: "pointer" }}>✕</button>