fix(analytics): put the Buy class on the masterclass basket forms too

Browser measurement on the live page: with plausible-event-name=Buy only on the
<button type="submit">, clicking it sends Plausible's automatic "Form: Submission"
event and no "Buy". With the class on the enclosing <form> the click sends exactly
one {"n":"Buy"}. The button keeps its class as specified; the form now carries it as
well, so the Buy goal fires once per click.

Same behaviour measured on the live bootcamp page (its three CTAs also emit only
"Form: Submission") - left untouched, reported as a site-wide follow-up.

odoo-n8n-masterclass/index.html

@@ -574,7 +574,7 @@
           <span class="hero-stat"><i aria-hidden="true">&#10003;</i> Certificate when you are done</span>
         </div>
         <div class="hero-actions">
-          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank">
+          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank" class="plausible-event-name=Buy">
             <input type="hidden" name="basket" value='[{"product_id":44,"qty":1}]' />
             <input type="hidden" name="clear" value="1" />
             <button type="submit" class="btn btn-brass btn-lg plausible-event-name=Buy">Buy for 120.00 USD</button>
@@ -599,7 +599,7 @@
         </p>
         <p class="bc-subline">Two hours. One working automation. Your system doing its own work.</p>
         <div class="bc-action">
-          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank">
+          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank" class="plausible-event-name=Buy">
             <input type="hidden" name="basket" value='[{"product_id":44,"qty":1}]' />
             <input type="hidden" name="clear" value="1" />
             <button type="submit" class="btn btn-brass btn-lg plausible-event-name=Buy">Buy for 120.00 USD</button>
@@ -720,7 +720,7 @@
           &mdash; calendar invites carry the correct local time.
         </p>
         <div class="bc-action">
-          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank">
+          <form method="post" action="https://20.odoo4projects.com/shop/post_basket" target="_blank" class="plausible-event-name=Buy">
             <input type="hidden" name="basket" value='[{"product_id":44,"qty":1}]' />
             <input type="hidden" name="clear" value="1" />
             <button type="submit" class="btn btn-brass btn-lg plausible-event-name=Buy">Buy for 120.00 USD</button>