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/postcss.config.js

@@ -1,6 +1,6 @@
 module.exports = {
   plugins: {
-    '@tailwindcss/postcss': {},
+    tailwindcss: {},
     autoprefixer: {},
   },
 }