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>

api.php

@@ -909,6 +909,35 @@ switch ($action) {
         }
         ok(['loggedIn' => false]);
 
+    // ─── Yhteydenotto ──────────────────────────────────────────
+    case 'contact':
+        $name    = trim($body['name'] ?? '');
+        $email   = trim($body['email'] ?? '');
+        $message = trim($body['message'] ?? '');
+
+        if (!$name || !$email || !$message) err('Täytä kaikki kentät.');
+        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) err('Sähköpostiosoite ei kelpaa.');
+        if (mb_strlen($message) > 5000) err('Viesti on liian pitkä.');
+
+        // Tallenna varmuuskopio
+        $messages = readData('messages.json', []);
+        $messages[] = [
+            'name'    => htmlspecialchars($name, ENT_QUOTES | ENT_HTML5, 'UTF-8'),
+            'email'   => htmlspecialchars($email, ENT_QUOTES | ENT_HTML5, 'UTF-8'),
+            'message' => htmlspecialchars($message, ENT_QUOTES | ENT_HTML5, 'UTF-8'),
+            'date'    => date('Y-m-d H:i:s'),
+        ];
+        writeData('messages.json', $messages);
+
+        // Lähetä sähköposti
+        $to      = 'info@tykkaa.fi';
+        $subject = "tykkää.fi: viesti käyttäjältä $name";
+        $body    = "Nimi: $name\nSähköposti: $email\n\nViesti:\n$message";
+        $headers = "From: noreply@tykkaa.fi\r\nReply-To: $email\r\nContent-Type: text/plain; charset=UTF-8";
+        @mail($to, $subject, $body, $headers);
+
+        ok(['sent' => true]);
+
     default:
         err('Unknown action');
 }