feat(FN-2686): improve system stats refresh UX on mobile

- Fix mobile refresh button icon visibility in SystemStatsModal
- Add auto-refresh activity indicator with spinning refresh state
- Update modal styles for responsive refresh controls and animation states
- Expand SystemStatsModal tests to cover mobile visibility and auto-refresh indicator behavior

packages/dashboard/app/components/SystemStatsModal.css

@@ -23,6 +23,44 @@
   align-items: center;
 }
 
+.system-stats-modal__auto-refresh {
+  display: inline-flex;
+  flex-direction: column;
+  align-items: flex-end;
+  color: var(--text-dim);
+  font-size: 0.8rem;
+  line-height: 1.25;
+}
+
+.system-stats-modal__auto-refresh-time {
+  color: var(--text-muted);
+  transition: color var(--transition-fast);
+}
+
+.system-stats-modal__header-actions .btn-icon {
+  flex-shrink: 0;
+}
+
+.system-stats-modal__header-actions .btn-icon svg {
+  display: block;
+  width: 1rem;
+  height: 1rem;
+}
+
+.system-stats-modal__refresh--spinning {
+  animation: system-stats-modal-refresh-spin var(--transition-slow) linear infinite;
+}
+
+@keyframes system-stats-modal-refresh-spin {
+  from {
+    transform: rotate(0deg);
+  }
+
+  to {
+    transform: rotate(360deg);
+  }
+}
+
 .system-stats-modal__state {
   margin: 0 var(--space-xl);
   padding: var(--space-md);
@@ -194,6 +232,21 @@
     margin-right: var(--space-lg);
   }
 
+  .system-stats-modal__auto-refresh {
+    align-items: flex-start;
+  }
+
+  .system-stats-modal__header-actions .btn-icon {
+    width: calc(var(--space-md) * 3);
+    height: calc(var(--space-md) * 3);
+    min-width: calc(var(--space-md) * 3);
+    min-height: calc(var(--space-md) * 3);
+    padding: 0;
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+  }
+
   .system-stats-modal__toggle-row,
   .system-stats-modal__threshold-row {
     align-items: flex-start;