feat: add top horizontal rail connecting header verticals

Adds a neon-green horizontal line across the very top of the site,
spanning from the left vertical rail to the right vertical rail, so
the header reads as a closed bracket instead of two floating lines.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/Header.tsx

@@ -244,6 +244,7 @@ export default function Header() {
 
   return (
     <div className="bb-header-rails relative">
+      <div className="bb-header-top-rail" aria-hidden />
       {/* TOP BAR */}
       <div className="max-w-container mx-auto px-4">
         <div className="bg-[#1a2535] border-b border-[#2a3a50] h-8 flex items-center justify-between">