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

docs/helix-stadium-concept.html

@@ -5,7 +5,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 <title>NAM Pedal — Helix Stadium UX Concept</title>
 <style>
-  @import url('https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;700&family=Inter:wght@300;400;500;600;700&display=swap');
+  @import url('https://fonts.googleapis.com/css2?family=JetBrains+Mono:wght@400;500;700&family=Space+Grotesk:wght@300;400;500;600;700&display=swap');
 
   * { box-sizing: border-box; margin: 0; padding: 0; user-select: none; -webkit-tap-highlight-color: transparent; }
 
@@ -29,7 +29,7 @@
     height: 100%;
     background: var(--bg);
     color: var(--text);
-    font-family: 'Inter', sans-serif;
+    font-family: 'Space Grotesk', sans-serif;
     overflow: hidden;
   }