feat: implement email notifications for contact form using nodemailer

[Adithyakp86]

Description

This Pull Request implements email notifications for the contact form submissions using the existing nodemailer dependency.

Currently, submissions are saved only to a local contacts.json file. With this change, a stylized HTML and raw text email notification is also sent to the site administrator when a new submission occurs.

Proposed Changes

• Nodemailer Integration: Imported nodemailer and configured an SMTP transporter utilizing environment variables (SMTP_HOST, SMTP_PORT, SMTP_SECURE, SMTP_USER, SMTP_PASS, CONTACT_SENDER, CONTACT_RECEIVER).
• Winston Logger Fallback: Implemented a fallback logger warning when SMTP credentials are not configured, printing the formatted notification details directly to the Winston logger. This prevents server crashes/errors in environments without SMTP configuration.
• Form Submission Hook: Updated the POST /contact endpoint to build the email payload and invoke the transporter asynchronously after successfully writing the entry to contacts.json.

Visual Design of the Email Notification

The notification email utilizes a clean, modern HTML layout featuring:

• Highlighted header with site theme styling (#667eea).
• Organized contact details (Name, Email link, Subject).
• Formatted message block with a blockquote indicator for enhanced readability.

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Verification & Testing

Manual Verification

1. Without SMTP Configured (Fallback):

   • Started the server locally (npm start).
   • Submitted a mock contact form submission.
   • Verified that the server outputted the fallback message:

     [warn]: SMTP credentials not configured. Email notifications will be logged instead of sent.

   • Verified the Winston logs printed the correct mail options payload:

     {
       "from": "no-reply@nexuscore.io",
       "to": "admin@nexuscore.io",
       "subject": "New Contact Form Submission: collaboration",
       "text": "Name: Jane Doe\nEmail: jane@example.com...",
       "html": "..."
     }

   • Confirmed the submission was successfully written to contacts.json.

2. Syntax Check:

   • Ran node -c server.js to ensure zero compilation or syntax errors are introduced.

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Checklist

• My code follows the code style of this project.
• I have verified the changes locally.
• All existing server logic and file writes operate as expected.