fix: Update Tailwind CSS and Better Auth imports for frontend build

- Update Tailwind CSS v4 PostCSS configuration
- Fix Better Auth React imports (better-auth/react)
- Update all page components to use Better Auth useSession
- Add @tailwindcss/postcss to dependencies
- Remove SessionProvider from layout (Better Auth handles sessions)
- Fix globals.css for Tailwind v4 compatibility
- Update page components with correct session state checking

This addresses build failures related to:
- Tailwind CSS v4 PostCSS plugin compatibility
- Better Auth React client import paths
- NextAuth v5 to Better Auth migration

frontend/app/globals.css

@@ -6,14 +6,16 @@
   :root {
     --background: 222.2 84% 4.9%;
     --foreground: 210 40% 98%;
+    --border: 217.2 32.6% 17.5%;
   }
 }
 
 @layer base {
   * {
-    @apply border-border;
+    border-color: hsl(var(--border));
   }
   body {
-    @apply bg-background text-foreground;
+    background-color: hsl(var(--background));
+    color: hsl(var(--foreground));
   }
 }