fix: improve masthead rail glow visibility

Replaced the invisible inset box-shadow glow (10px blur inside a 3px
element) with a white overlay that pulses opacity 0.15-0.55 over 4s.
Sped up glow animation from 5.5s to 4s for a more noticeable breath.

frontend/src/components/ui/MastheadRail.tsx

@@ -16,7 +16,7 @@ export function MastheadRail({ variant, className }: MastheadRailProps) {
             {variant === 'shimmer' ? (
                 <div className="masthead-rail-shimmer absolute inset-x-0 h-1/3 bg-gradient-to-b from-transparent via-white/50 to-transparent" />
             ) : (
-                <div className="masthead-rail-glow absolute inset-0" />
+                <div className="masthead-rail-glow absolute inset-0 bg-white/20" />
             )}
         </div>
     );