refactor: replace pixel-based TOC sidebar width and sticky offset with flexible values

app/posts/[slug]/page.tsx

@@ -36,7 +36,7 @@ export default async function PostPage({ params }: { params: Promise<{ slug: str
       <ScrollToTop />
       <ReadingProgress />
       <div className="mx-auto max-w-6xl px-6 py-16">
-        <div className="grid grid-cols-1 gap-8 lg:grid-cols-[minmax(0,1fr)_220px] lg:gap-12">
+        <div className="grid grid-cols-1 gap-8 lg:grid-cols-[minmax(0,1fr)_14rem] lg:gap-12">
           <article className="min-w-0">
             <header className="mb-12">
               <time className="font-mono text-sm text-ink-soft" dateTime={post.date}>{new Date(post.date).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' })}</time>

components/posts/TableOfContents.tsx

@@ -74,7 +74,7 @@ export function TableOfContents() {
   if (headings.length === 0) return null;
 
   return (
-    <nav aria-label="Table of contents" className="lg:sticky lg:top-[var(--header-height)]">
+    <nav aria-label="Table of contents" className="lg:sticky lg:top-20">
       <h4 className="font-sans text-xs font-semibold uppercase tracking-wider text-ink-soft mb-3">
         On this page
       </h4>