Promote admin join code display

frontend/src/assets/css/admin.css

@@ -23,6 +23,58 @@
     font-size: 0.875rem;
 }
 
+.admin-summary {
+    display: flex;
+    align-items: flex-start;
+    justify-content: space-between;
+    gap: 1.5rem;
+}
+
+.admin-summary-details {
+    min-width: 0;
+}
+
+.admin-join-code {
+    flex: 0 0 auto;
+    min-width: 10.5rem;
+    padding: 0.85rem 1.1rem 0.95rem;
+    background: color-mix(in srgb, var(--accent) 12%, var(--well));
+    border: 2px solid var(--accent);
+    border-radius: var(--radius-md);
+    box-shadow: var(--shadow-soft);
+    text-align: center;
+}
+
+.admin-join-code-label {
+    display: block;
+    margin-bottom: 0.2rem;
+    color: var(--text-muted);
+    font-family: var(--font-heading);
+    font-size: 0.75rem;
+    font-weight: 700;
+    letter-spacing: 0.12em;
+    text-transform: uppercase;
+}
+
+.admin-join-code strong {
+    display: block;
+    color: var(--accent);
+    font-family: ui-monospace, monospace;
+    font-size: 2rem;
+    line-height: 1;
+    letter-spacing: 0.16em;
+}
+
+@media (max-width: 600px) {
+    .admin-summary {
+        flex-direction: column;
+    }
+
+    .admin-join-code {
+        width: 100%;
+    }
+}
+
 .admin-table {
     width: 100%;
     text-align: left;