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>

tailwind.config.js

@@ -41,7 +41,7 @@ export default {
       },
       fontFamily: {
         sans: [
-          "Inter",
+          "Geist",
           "system-ui",
           "-apple-system",
           "BlinkMacSystemFont",