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/portfolio/page.tsx

@@ -1,6 +1,6 @@
 'use client';
 
-import { useSession } from 'next-auth/react';
+import { useSession } from '@/lib/better-auth';
 import { useRouter } from 'next/navigation';
 import { useEffect, useState } from 'react';
 import { LineChart, Line, XAxis, YAxis, CartesianGrid, Tooltip, ResponsiveContainer, PieChart, Pie, Cell, Legend } from 'recharts';
@@ -8,7 +8,7 @@ import { format } from 'date-fns';
 import Link from 'next/link';
 
 export default function PortfolioPage() {
-  const { data: session, status } = useSession();
+  const { data: session, isPending } = useSession();
   const router = useRouter();
   const [portfolio, setPortfolio] = useState([]);
   const [summary, setSummary] = useState({ total_value: 0, total_gain_loss: 0, cost_basis: 0 });
@@ -17,7 +17,7 @@ export default function PortfolioPage() {
   const [newHolding, setNewHolding] = useState({ ticker: '', shares: '', avg_cost: '' });
 
   useEffect(() => {
-    if (status === 'unauthenticated') {
+    if (!isPending && !session) {
       router.push('/auth/signin');
       return;
     }
@@ -25,7 +25,7 @@ export default function PortfolioPage() {
     if (session?.user) {
       fetchPortfolio(session.user.id);
     }
-  }, [session, status, router]);
+  }, [session, isPending, router]);
 
   const fetchPortfolio = async (userId: string) => {
     try {