fix: move creation loading indicator to create session form

Move the loading overlay from the active sessions section to the create
session section so it dims the form itself during creation, providing
better visual feedback to the user.

apps/web/src/styles.css

@@ -2825,9 +2825,15 @@ a.nav-item,
 }
 
 .create-session-section {
+  position: relative;
   margin-bottom: var(--space-6);
 }
 
+.create-session-section.dimmed {
+  opacity: 0.5;
+  pointer-events: none;
+}
+
 .create-session-form {
   max-width: 600px;
 }