integration: disable the contact mailto until required fields are complete

Keep the submit control focusable with a disabled semantic while the briefing is incomplete, mark invalid fields only after interaction, and leave the encoded mailto path unchanged once the form is filled.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/app/shared/contact-briefing/contact-briefing.scss

@@ -31,7 +31,8 @@ textarea {
   font: inherit;
 }
 
-.actions a {
+.actions a,
+button.submit {
   color: var(--color-accent-cool);
   text-decoration: none;
 }
@@ -40,6 +41,21 @@ textarea {
   font-weight: 600;
 }
 
+button.submit {
+  appearance: none;
+  border: 0;
+  padding: 0;
+  background: transparent;
+  font: inherit;
+  font-weight: 600;
+  min-height: 2.75rem;
+}
+
+button.submit[aria-disabled='true'] {
+  color: var(--color-text-muted);
+  cursor: not-allowed;
+}
+
 @media (hover: hover) and (pointer: fine) {
   .actions a:hover {
     text-decoration: underline;