Fix session cookie size issue by implementing server-side sessions with Flask-Session

ubuntu-install.sh

@@ -328,6 +328,11 @@ sudo chown $USER:$USER /var/log/pdf-sort-merge
 log_info "Creating application data directories..."
 mkdir -p results static templates
 
+# Create Flask session directory for server-side sessions
+log_info "Creating Flask session directory..."
+sudo mkdir -p /tmp/flask_sessions
+sudo chown $USER:$USER /tmp/flask_sessions
+
 # Set up firewall (optional)
 log_info "Configuring firewall..."
 sudo ufw allow 80/tcp || log_warning "Failed to configure firewall for port 80 - you may need to do this manually"