feat: implement email sending functionality with SMTP and Resend fallback

- Added support for sending emails using SMTP configuration or Resend API as a fallback.
- Updated environment variables in .env_example for SMTP settings.
- Enhanced email confirmation process with improved error handling and fallback logic.
- Introduced a test script to validate email sending configuration and functionality.

viewer-frontend/package-lock.json

@@ -27,6 +27,7 @@
         "lucide-react": "^0.553.0",
         "next": "^16.1.1",
         "next-auth": "^5.0.0-beta.30",
+        "nodemailer": "^7.0.13",
         "prisma": "^6.19.0",
         "react": "19.2.0",
         "react-day-picker": "^9.11.1",
@@ -40,6 +41,7 @@
       "devDependencies": {
         "@tailwindcss/postcss": "^4",
         "@types/node": "^20",
+        "@types/nodemailer": "^7.0.11",
         "@types/react": "^19",
         "@types/react-dom": "^19",
         "dotenv": "^17.2.3",
@@ -2021,6 +2023,16 @@
         "undici-types": "~6.21.0"
       }
     },
+    "node_modules/@types/nodemailer": {
+      "version": "7.0.11",
+      "resolved": "https://registry.npmjs.org/@types/nodemailer/-/nodemailer-7.0.11.tgz",
+      "integrity": "sha512-E+U4RzR2dKrx+u3N4DlsmLaDC6mMZOM/TPROxA0UAPiTgI0y4CEFBmZE+coGWTjakDriRsXG368lNk1u9Q0a2g==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/@types/react": {
       "version": "19.2.7",
       "devOptional": true,
@@ -6116,6 +6128,15 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/nodemailer": {
+      "version": "7.0.13",
+      "resolved": "https://registry.npmjs.org/nodemailer/-/nodemailer-7.0.13.tgz",
+      "integrity": "sha512-PNDFSJdP+KFgdsG3ZzMXCgquO7I6McjY2vlqILjtJd0hy8wEvtugS9xKRF2NWlPNGxvLCXlTNIae4serI7dinw==",
+      "license": "MIT-0",
+      "engines": {
+        "node": ">=6.0.0"
+      }
+    },
     "node_modules/nopt": {
       "version": "9.0.0",
       "resolved": "https://registry.npmjs.org/nopt/-/nopt-9.0.0.tgz",