fix: use TierName type + validation in TopNav for consistency with DashboardHome

Author: DevanshuNEU

frontend/src/components/dashboard/TopNav.tsx

@@ -1,5 +1,6 @@
 import { Link } from 'react-router-dom'
 import { useAuth } from '../../contexts/AuthContext'
+import { TIER_FUNCTION_LIMITS, type TierName } from '../../config/api'
 import { Menu, Search, Github, Sun, Moon, LogOut, Settings, BookOpen, ExternalLink } from 'lucide-react'
 import { useTheme } from 'next-themes'
 import { Button } from '@/components/ui/button'
@@ -24,7 +25,8 @@ export function TopNav({ onToggleSidebar, sidebarCollapsed, onOpenCommandPalette
 
   const userEmail = session?.user?.email || 'User'
   const userInitial = userEmail.charAt(0).toUpperCase()
-  const userTier = (session?.user?.user_metadata?.tier as string) || 'free'
+  const rawTier = session?.user?.user_metadata?.tier as string
+  const userTier: TierName = rawTier && rawTier in TIER_FUNCTION_LIMITS ? (rawTier as TierName) : 'free'
   const tierLabel = `${userTier.charAt(0).toUpperCase()}${userTier.slice(1)} Plan`
 
   const toggleTheme = () => {