feat: add admin layout with sub-navigation and routing

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

src/pages/Admin/Admin.module.css

@@ -0,0 +1,36 @@
+.adminNav {
+  display: flex;
+  gap: 0;
+  border-bottom: 1px solid var(--border-subtle);
+  padding: 0 20px;
+  background: var(--bg-base);
+}
+
+.adminTab {
+  padding: 10px 16px;
+  border: none;
+  background: none;
+  color: var(--text-secondary);
+  font-family: var(--font-body);
+  font-size: 13px;
+  font-weight: 500;
+  cursor: pointer;
+  border-bottom: 2px solid transparent;
+  margin-bottom: -1px;
+  transition: color 0.15s, border-color 0.15s;
+}
+
+.adminTab:hover {
+  color: var(--text-primary);
+}
+
+.adminTabActive {
+  color: var(--amber);
+  border-bottom-color: var(--amber);
+}
+
+.adminContent {
+  flex: 1;
+  overflow-y: auto;
+  padding: 20px;
+}