feat: migrar emails de Resend a Nodemailer + Gmail SMTP

Reemplaza Resend por nodemailer con Gmail SMTP. El to: ahora usa
el email real del cliente en lugar del hardcodeado de sandbox.
Variables de entorno: GMAIL_USER y GMAIL_APP_PASSWORD.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -18,15 +18,16 @@
   "dependencies": {
     "@prisma/adapter-pg": "^7.8.0",
     "@prisma/client": "^7.8.0",
+    "@types/nodemailer": "^8.0.1",
     "bcryptjs": "^3.0.3",
     "jose": "^6.2.3",
     "jsonwebtoken": "^9.0.3",
     "next": "14.2.35",
     "next-auth": "^5.0.0-beta.25",
+    "nodemailer": "^6.10.1",
     "prisma": "^7.8.0",
     "react": "^18",
-    "react-dom": "^18",
-    "resend": "^6.14.0"
+    "react-dom": "^18"
   },
   "devDependencies": {
     "@types/bcryptjs": "^2.4.6",