fix: show empty tier label while loading instead of 'Free Plan' flash

Enterprise users saw 'Free Plan' briefly before useUserUsage resolved.
Now shows empty string until backend responds with actual tier.

Author: DevanshuNEU

frontend/src/components/dashboard/TopNav.tsx

@@ -26,8 +26,9 @@ export function TopNav({ onToggleSidebar, sidebarCollapsed, onOpenCommandPalette
   const userEmail = session?.user?.email || 'User'
   const userInitial = userEmail.charAt(0).toUpperCase()
   const { data: usage } = useUserUsage(session?.access_token, session?.user?.id)
-  const tier = usage?.tier || 'free'
-  const tierLabel = `${tier.charAt(0).toUpperCase()}${tier.slice(1)} Plan`
+  const tierLabel = usage?.tier
+    ? `${usage.tier.charAt(0).toUpperCase()}${usage.tier.slice(1)} Plan`
+    : ''
 
   const toggleTheme = () => {
     setTheme(theme === 'dark' ? 'light' : 'dark')