Switch to Geist font family

Replace Inter with Geist font from jsDelivr CDN for a more modern look
that aligns with Vercel's design system.

Changes:
- index.html: Add Geist font stylesheet from jsDelivr
- main.css: Update font-family to use "Geist" as primary
- tailwind.config.js: Update fontFamily.sans to start with "Geist"

Geist is Vercel's open-source font family designed for readability and
clarity in developer tools. It pairs well with JetBrains Mono for code
blocks (already in use).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/client/styles/main.css

@@ -36,11 +36,10 @@
     --color-glow-accent: rgba(91, 156, 245, 0.12);
     --color-glow-success: rgba(63, 185, 80, 0.12);
 
-    /* Inter font from Google Fonts CDN */
-    font-family: "Inter", system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
+    /* Geist font from jsDelivr CDN */
+    font-family: "Geist", system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
     -webkit-font-smoothing: antialiased;
     -moz-osx-font-smoothing: grayscale;
-    font-feature-settings: "cv02", "cv03", "cv04", "cv11";
   }
 
   /* Smooth transitions on all interactive elements */