feat(design-system): add loading.tsx route transition page

web/src/app/loading.tsx

@@ -0,0 +1,13 @@
+export default function Loading() {
+  return (
+    <div className="flex min-h-screen items-center justify-center" style={{ background: 'var(--cm-bg-canvas, #06070A)' }}>
+      <div className="flex flex-col items-center gap-3">
+        <div
+          className="h-8 w-8 animate-spin rounded-full border-2 border-t-transparent"
+          style={{ borderColor: 'var(--cm-accent, #5A8CFF)', borderTopColor: 'transparent' }}
+        />
+        <p className="text-sm" style={{ color: 'var(--cm-text-secondary, #A5B1C7)' }}>Loading...</p>
+      </div>
+    </div>
+  );
+}