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/filings/page.tsx

@@ -1,20 +1,20 @@
 '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 Link from 'next/link';
 import { format } from 'date-fns';
 
 export default function FilingsPage() {
-  const { data: session, status } = useSession();
+  const { data: session, isPending } = useSession();
   const router = useRouter();
   const [filings, setFilings] = useState([]);
   const [loading, setLoading] = useState(true);
   const [searchTicker, setSearchTicker] = useState('');
 
   useEffect(() => {
-    if (status === 'unauthenticated') {
+    if (!isPending && !session) {
       router.push('/auth/signin');
       return;
     }
@@ -22,7 +22,7 @@ export default function FilingsPage() {
     if (session?.user) {
       fetchFilings();
     }
-  }, [session, status, router]);
+  }, [session, isPending, router]);
 
   const fetchFilings = async (ticker?: string) => {
     setLoading(true);