feat: Swap display font from Plus Jakarta Sans to Instrument Serif

Plus Jakarta Sans was too similar to Geist Sans — invisible difference.
Instrument Serif creates strong serif/sans contrast for headings, making
typographic hierarchy immediately obvious.

apps/web/index.html

@@ -6,7 +6,7 @@
     <title>Codewalk District</title>
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-    <link href="https://fonts.googleapis.com/css2?family=Plus+Jakarta+Sans:wght@500;600;700;800&display=swap" rel="stylesheet">
+    <link href="https://fonts.googleapis.com/css2?family=Instrument+Serif:ital@0;1&display=swap" rel="stylesheet">
     <script>
       (function() {
         var t = localStorage.getItem('cw-theme') || 'system';

apps/web/tailwind.config.ts

@@ -18,7 +18,7 @@ export default {
       fontFamily: {
         sans: ["Geist Sans", ...defaultTheme.fontFamily.sans],
         mono: ["Geist Mono", ...defaultTheme.fontFamily.mono],
-        display: ['"Plus Jakarta Sans"', "var(--font-geist-sans)", "system-ui", "sans-serif"],
+        display: ['"Instrument Serif"', "Georgia", "serif"],
       },
       colors: {
         border: "hsl(var(--border))",