feat(ui): unify loading empty error states

frontend/src/styles/main.css

@@ -437,6 +437,45 @@ body {
   font-weight: 600;
 }
 
+/* Unified loading / empty / error states */
+.ctms-state {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 8px;
+  padding: 28px 16px;
+  text-align: center;
+  border: 1px solid var(--ctms-border-color);
+  border-radius: var(--ctms-radius);
+  background: #ffffff;
+}
+
+.ctms-state-icon {
+  font-size: 34px;
+}
+
+.ctms-state-title {
+  margin: 0;
+  font-size: 15px;
+  font-weight: 600;
+}
+
+.ctms-state-desc {
+  margin: 0;
+  font-size: 13px;
+  color: var(--ctms-text-secondary);
+}
+
+.ctms-state-action {
+  margin-top: 4px;
+}
+
+.ctms-state-loading .ctms-state-skeleton {
+  width: min(560px, 100%);
+  margin-top: 4px;
+}
+
 /* Tool classes */
 .text-main {
   color: var(--ctms-text-main);