fix: reduce padding and spacing for mobile

Main: px-4 py-6 on mobile (was px-8 py-12). Section accordion:
mb/py on inactive links tightened to 1/1 on mobile, space-y-0
between sections. Active title text-xl on mobile to prevent
wrapping at ~400px, matches inactive title size.

app/[[...slug]]/page.tsx

@@ -41,7 +41,7 @@ export default async function SectionPage({ params }: Props) {
   }
 
   return (
-    <main className="px-8 py-12 lg:py-16 lg:px-16">
+    <main className="px-4 py-6 sm:px-8 sm:py-12 lg:py-16 lg:px-16">
       <SectionsAccordion sections={sections} activeSlug={activeSlug}>
         {sections.map((s) => (
           <SectionFactory key={s.slug} slug={s.slug} />