Restore contact form and make it functional with email sending

- Replace mailto link with original contact form (name, email, message fields)
- Add contact API endpoint that sends email via mail() and saves to messages.json
- Restore .contact-form CSS styles and translation keys

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

index.html

@@ -64,7 +64,12 @@
     <div class="container">
       <h2 data-i18n="contact_title"></h2>
       <p data-i18n="contact_desc"></p>
-      <a href="mailto:info@tykkaa.fi" class="contact-email">info@tykkaa.fi</a>
+      <form class="contact-form" onsubmit="handleSubmit(event)">
+        <input type="text" id="contact-name" data-i18n-ph="name_ph" required />
+        <input type="email" id="contact-email" data-i18n-ph="email_ph" required />
+        <textarea id="contact-msg" data-i18n-ph="msg_ph" rows="4" required></textarea>
+        <button type="submit" class="btn" data-i18n="send_btn"></button>
+      </form>
     </div>
   </section>