feat(#112): add admin header bar with back button and logout

AdminLayout gains a self-contained header (Back / Admin / user+logout)
with CSS module styles, replacing the inline padding wrapper. Admin
pages now render fully without the main app chrome.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/pages/Admin/AdminLayout.module.css

@@ -0,0 +1,69 @@
+.header {
+  display: flex;
+  align-items: center;
+  height: 48px;
+  padding: 0 16px;
+  background: var(--bg-surface);
+  border-bottom: 1px solid var(--border-subtle);
+}
+
+.backBtn {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  background: none;
+  border: none;
+  color: var(--text-secondary);
+  font-size: 13px;
+  font-family: var(--font-body);
+  cursor: pointer;
+  padding: 6px 10px;
+  border-radius: var(--radius-sm);
+}
+
+.backBtn:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.title {
+  flex: 1;
+  text-align: center;
+  font-size: 12px;
+  font-weight: 600;
+  letter-spacing: 0.08em;
+  color: var(--text-muted);
+  text-transform: uppercase;
+}
+
+.userSection {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+}
+
+.username {
+  font-size: 13px;
+  color: var(--text-secondary);
+  font-family: var(--font-body);
+}
+
+.logoutBtn {
+  background: none;
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-sm);
+  padding: 4px 12px;
+  font-size: 12px;
+  color: var(--text-secondary);
+  cursor: pointer;
+  font-family: var(--font-body);
+}
+
+.logoutBtn:hover {
+  background: var(--bg-hover);
+  color: var(--text-primary);
+}
+
+.content {
+  padding: 20px 24px 40px;
+}