style: tint topbar "Built with hubris" footer to black/50

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/TopBar.tsx

@@ -108,7 +108,7 @@ export function TopBar({
           </button>
         )}
         <span
-          className="text-[10px] text-foreground-muted"
+          className="text-[10px] text-black/50"
           style={{ textShadow: '0 1px 2px rgba(0,0,0,0.7)' }}
         >
           Built with hubris • {toRoman(new Date().getFullYear())}