style(frontend): refactor BottomNav for DESIGN.md compliance

- Used 'level-0' and 'border-border' instead of hardcoded hex codes
- Removed 'font-sans' in favor of global font setting

frontend/components/BottomNav.tsx

@@ -25,7 +25,7 @@ export default function BottomNav({
   const isAdmin = pathname === '/admin';
 
   return (
-    <footer className="fixed bottom-0 left-0 right-0 py-3 lg:py-4 xl:py-5 px-2 pb-safe bg-[#0A0A0A] border-t border-[#222222] z-40 transition-all duration-300 font-sans">
+    <footer className="fixed bottom-0 left-0 right-0 py-3 lg:py-4 xl:py-5 px-2 pb-safe level-0 border-t border-border z-40 transition-all duration-300">
       <div className="max-w-xl mx-auto flex justify-around items-center text-[#888888]">
         
         <button