style(landing): center the What is OpenTaint? and Why is OpenTaint? headings

Center just the two heading phrases; the subheadline, intro lines, and bullets
stay left-aligned within the centered box.

Author: misonijnik

src/components/astro/WhatIsOpenTaint.astro

@@ -20,12 +20,12 @@ const features = [
 
 <section class="band band-tint" aria-labelledby="what-heading">
   <div class="mx-auto max-w-5xl">
-    <h2 id="what-heading" class="section-title">What is OpenTaint?</h2>
+    <h2 id="what-heading" class="section-title text-center">What is OpenTaint?</h2>
     <p class="subheadline">
       Formal taint analysis for application security — finds what AST-pattern matchers miss, lets LLM agents enact vulnerabilities as rules, scales where neither can alone.
     </p>
 
-    <h2 id="why-heading" class="section-title mt-12 lg:mt-14">Why is OpenTaint?</h2>
+    <h2 id="why-heading" class="section-title mt-12 lg:mt-14 text-center">Why is OpenTaint?</h2>
     <p class="section-subtitle">
       AI generates production code faster than today's security tooling can keep up with.
     </p>