fix(contact): use Formspree's reserved _subject field for the subject line

The hidden field was named `subject`, which Formspree treats as ordinary form
data: notifications would have arrived under a generic "New submission"
subject with "subject" showing up as just another row in the body. The
reserved field is `_subject`, with the underscore.

build 0/0, contact e2e 4 passed.

src/pages/contact.astro

@@ -34,7 +34,9 @@ const endpoint = import.meta.env.PUBLIC_CONTACT_FORM_ENDPOINT ?? "";
         Tell us about your project and we will follow up within two business days.
       </p>
       <form class="contact-form" method="post" action={endpoint || "/thank-you/"} data-contact-form data-endpoint={endpoint}>
-        <input type="hidden" name="subject" value="BlueCap Strategies website inquiry" />
+        {/* `_subject` (underscore) is Formspree's reserved field for the notification subject
+            line. Without it the mail arrives under a generic "New submission" subject. */}
+        <input type="hidden" name="_subject" value="BlueCap Strategies website inquiry" />
         {/* Formspree treats a filled _gotcha field as spam. Hidden from people, not from bots. */}
         <input type="text" name="_gotcha" tabindex="-1" autocomplete="off" aria-hidden="true" class="gotcha" />
         <div class="field-grid">