Update UI components and styling

This commit includes overall UI and styling improvements in different areas of the application. In particular, it refactors the font usage in the tailwind configuration and modifies the Heading import path in `heading.tsx`. The pricing table and line item details in the billing gateway had been extensively updated to provide better visual representation. Finally, changes were made to the marketing page to make it more straightforward and clear.

tooling/tailwind/index.ts

@@ -64,7 +64,7 @@ export default {
       },
       fontFamily: {
         cal: ['var(--font-cal)', ...fontFamily.sans],
-        sans: ['system-ui', 'var(--font-sans)', ...fontFamily.sans],
+        sans: ['-apple-system', 'var(--font-sans)', ...fontFamily.sans],
         heading: ['var(--font-heading)'],
       },
       keyframes: {