Group Back-to-project and Manage together, styled alike

Moves the back link from the far left to sit immediately left of the
Manage button (both now top right, in a new .project-head__actions
row), and switches it from .btn-secondary to .menu__toggle so the two
are visually identical rather than just similar.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/style.css

@@ -598,10 +598,16 @@ h1 {
   background: var(--bg);
 }
 
+.project-head__actions {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  flex: none;
+}
+
 .project-head__back {
   display: inline-flex;
   align-items: center;
-  flex: none;
   text-decoration: none;
 }