Add light mode support (next-themes), JetBrains Mono font, OpenType features

- layout.tsx: ThemeProvider from next-themes, light mode DEFAULT, JetBrains_Mono font
- globals.css: font-mono pointing to correct variable, font-feature-settings cv01+ss03 on body
- next-themes package installed
- Build passes with zero errors

web/package.json

@@ -17,6 +17,7 @@
     "date-fns": "^4.4.0",
     "lucide-react": "^1.17.0",
     "next": "16.2.7",
+    "next-themes": "^0.4.6",
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "react-hook-form": "^7.77.0",