feat(modals): FormModal gains submitDisabled + subtitle props

submitDisabled greys out the primary (so callers can gate it on validity
instead of a clickable-but-inert button); subtitle renders a muted line under
the title in the header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/components.css

@@ -255,12 +255,23 @@
   gap: 12px;
 }
 
+.admin-modal-heading {
+  min-width: 0;
+}
+
 .admin-modal-title {
   font-size: 16px;
   font-weight: 700;
   color: var(--text-primary);
 }
 
+.admin-modal-subtitle {
+  margin: 2px 0 0;
+  font-size: 12.5px;
+  font-weight: 500;
+  color: var(--text-secondary);
+}
+
 .admin-modal-close {
   flex-shrink: 0;
   display: inline-flex;