fix: eliminate pull-to-refresh flicker by holding stale data during re-fetch

lib/app.dart

@@ -236,13 +236,13 @@ class _ShellState extends ConsumerState<_Shell> with WidgetsBindingObserver {
   /// Refresh every major data provider. Safe to call speculatively —
   /// providers that aren't currently watched are already disposed.
   void _refreshAll() {
-    ref.invalidate(conversationsProvider);
-    ref.invalidate(calendarProvider);
-    ref.invalidate(newsProvider);
-    // Notifier (not AsyncNotifier) — needs explicit refresh call.
+    ref.read(conversationsProvider.notifier).refresh();
+    ref.read(calendarProvider.notifier).refresh();
+    ref.read(newsProvider.notifier).refresh();
     ref.read(knowledgeProvider.notifier).refresh();
-    // briefingProvider is an AsyncNotifier family; invalidating the family
-    // is safe even if no conversation is open.
+    // briefingProvider is an AsyncNotifier family; invalidating is safe
+    // even if no conversation is open — it doesn't cause flicker since
+    // the briefing screen isn't a list view.
     ref.invalidate(briefingProvider);
   }
 
@@ -254,7 +254,7 @@ class _ShellState extends ConsumerState<_Shell> with WidgetsBindingObserver {
       case 1:
         ref.read(knowledgeProvider.notifier).refresh();
       case 2:
-        ref.invalidate(conversationsProvider);
+        ref.read(conversationsProvider.notifier).refresh();
     }
   }