fix: submit email form via AJAX instead of redirect

Submits to Formspree via fetch so the user stays on the page.
Shows green success or red error message below the form.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: savvides

docs/index.html

@@ -556,7 +556,10 @@
       color: var(--text-muted);
       text-align: center;
       margin-top: 0.6rem;
+      transition: color 0.3s;
     }
+    .signup-note.success { color: var(--terminal-green); }
+    .signup-note.error { color: #ef4444; }
 
     /* Builder note */
     .builder-note {
@@ -633,12 +636,11 @@ <h1>
       <span class="gradient">Design better courses.</span>
     </h1>
     <p>Open source instructional design skills for Claude Code. Every recommendation is backed by peer-reviewed research, so you always know how strong the evidence is.</p>
-    <form action="https://formspree.io/f/xkokkzyy" method="POST" class="signup-form">
+    <form id="signup-form" class="signup-form">
       <input type="email" name="email" placeholder="your@email.com" required aria-label="Email address">
       <button type="submit">Get updates</button>
-      <input type="hidden" name="_next" value="https://idstack.org/">
     </form>
-    <p class="signup-note">No spam. Just releases.</p>
+    <p class="signup-note" id="signup-note">No spam. Just releases.</p>
   </header>
 
   <hr class="section-divider" style="margin-top: 3rem;">
@@ -1040,6 +1042,35 @@ <h3>Where we're headed</h3>
         document.body.removeChild(textarea);
       }
     }
+
+    document.getElementById('signup-form').addEventListener('submit', function(e) {
+      e.preventDefault();
+      var form = this;
+      var btn = form.querySelector('button');
+      var note = document.getElementById('signup-note');
+      btn.disabled = true;
+      btn.textContent = 'Sending...';
+      fetch('https://formspree.io/f/xkokkzyy', {
+        method: 'POST',
+        body: new FormData(form),
+        headers: { 'Accept': 'application/json' }
+      }).then(function(res) {
+        if (res.ok) {
+          note.textContent = 'You\u2019re in! We\u2019ll keep you posted.';
+          note.className = 'signup-note success';
+          form.reset();
+        } else {
+          note.textContent = 'Something went wrong. Try again.';
+          note.className = 'signup-note error';
+        }
+      }).catch(function() {
+        note.textContent = 'Something went wrong. Try again.';
+        note.className = 'signup-note error';
+      }).finally(function() {
+        btn.disabled = false;
+        btn.textContent = 'Get updates';
+      });
+    });
   </script>
 
 </body>