feat(env): Update environment configuration and enhance email functionality

- Renamed DATABASE_URL to POSTGRES_URL in .env.example for clarity.
- Added SMTP configuration for email sending, including host, port, user, password, and from address.
- Updated package.json to include new dependencies for email handling and UI components.
- Modified middleware to handle public and protected routes more effectively.
- Enhanced API routes for studies to support user roles and permissions.
- Updated database schema to include invitations and user roles related to studies.
- Improved user permissions handling in the permissions module.
- Added new utility functions for managing user roles and study access.

src/lib/email.ts

@@ -0,0 +1,76 @@
+import nodemailer from 'nodemailer';
+
+// Create reusable transporter object using SMTP transport
+const transporter = nodemailer.createTransport({
+  service: 'iCloud',
+  secure: false,
+  auth: {
+    user: 'soconnor0919@icloud.com',
+    pass: 'uhlb-virv-qqpk-puwc',
+  },
+});
+
+// Verify connection configuration
+transporter.verify(function(error, success) {
+  if (error) {
+    console.log('SMTP Verification Error:', error);
+  }
+});
+
+interface SendInvitationEmailParams {
+  to: string;
+  inviterName: string;
+  studyTitle: string;
+  role: string;
+  token: string;
+}
+
+export async function sendInvitationEmail({
+  to,
+  inviterName,
+  studyTitle,
+  role,
+  token,
+}: SendInvitationEmailParams) {
+  const inviteUrl = `${process.env.NEXT_PUBLIC_APP_URL}/invite/accept/${token}`;
+
+  const html = `
+    <h2>You've been invited to join HRIStudio</h2>
+    <p>${inviterName} has invited you to join their study "${studyTitle}" as a ${role}.</p>
+    <p>HRIStudio is a platform for managing human-robot interaction studies and Wizard-of-Oz experiments.</p>
+    <p>Click the button below to accept the invitation and join the study:</p>
+    <a href="${inviteUrl}" style="
+      display: inline-block;
+      background-color: #0070f3;
+      color: white;
+      padding: 12px 24px;
+      text-decoration: none;
+      border-radius: 6px;
+      margin: 16px 0;
+    ">Accept Invitation</a>
+    <p>Or copy and paste this URL into your browser:</p>
+    <p>${inviteUrl}</p>
+    <p>This invitation will expire in 7 days.</p>
+  `;
+
+  const text = `
+You've been invited to join HRIStudio
+
+${inviterName} has invited you to join their study "${studyTitle}" as a ${role}.
+
+HRIStudio is a platform for managing human-robot interaction studies and Wizard-of-Oz experiments.
+
+To accept the invitation, visit this URL:
+${inviteUrl}
+
+This invitation will expire in 7 days.
+  `;
+
+  await transporter.sendMail({
+    from: `"HRIStudio" <${process.env.SMTP_FROM_ADDRESS}>`,
+    to,
+    subject: `Invitation to join "${studyTitle}" on HRIStudio`,
+    text,
+    html,
+  });
+}