feat(ui): make header sticky with glassy backdrop blur

Add sticky positioning and glassmorphism effect to the top navigation:
- sticky top-0 z-50 for fixed positioning above content
- bg-card/80 for 80% opacity background
- backdrop-blur-md for frosted glass effect

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ui/src/App.tsx

@@ -242,7 +242,7 @@ function App() {
   return (
     <div className="min-h-screen bg-background">
       {/* Header */}
-      <header className="bg-card text-foreground border-b-2 border-border">
+      <header className="sticky top-0 z-50 bg-card/80 backdrop-blur-md text-foreground border-b-2 border-border">
         <div className="max-w-7xl mx-auto px-4 py-4">
           <div className="flex items-center justify-between">
             {/* Logo and Title */}