feat(mui): kit primitives — PageHeader, EmptyState, LoadingState, FilterBar, StatCard + DataTable onRowClick/rowDanger

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/LoadingState.tsx

@@ -0,0 +1,30 @@
+import Box from "@mui/material/Box";
+import CircularProgress from "@mui/material/CircularProgress";
+import Typography from "@mui/material/Typography";
+
+export interface LoadingStateProps {
+  label?: string;
+}
+
+/** Centered loading indicator — replaces admin-loading/admin-spinner CSS patterns. */
+export default function LoadingState({ label }: LoadingStateProps) {
+  return (
+    <Box
+      sx={{
+        display: "flex",
+        flexDirection: "column",
+        alignItems: "center",
+        justifyContent: "center",
+        py: 8,
+        gap: 2,
+      }}
+    >
+      <CircularProgress size={36} />
+      {label && (
+        <Typography variant="body2" color="text.secondary">
+          {label}
+        </Typography>
+      )}
+    </Box>
+  );
+}