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

lib/screens/calendar/calendar_screen.dart

@@ -68,7 +68,7 @@ class CalendarScreen extends ConsumerWidget {
             const Divider(height: 1),
             Expanded(
               child: RefreshIndicator(
-                onRefresh: () async => ref.invalidate(calendarProvider),
+                onRefresh: () => ref.read(calendarProvider.notifier).refresh(),
                 child: _AgendaList(
                   events:
                       cal.eventsByDay[dateOnly(cal.selectedDay)] ?? [],

lib/screens/chat/chat_screen.dart

@@ -30,7 +30,7 @@ class _ChatScreenState extends ConsumerState<ChatScreen>
   @override
   void didChangeAppLifecycleState(AppLifecycleState state) {
     if (state == AppLifecycleState.resumed) {
-      ref.invalidate(messagesProvider(widget.conversationId));
+      ref.read(messagesProvider(widget.conversationId).notifier).refresh();
     }
   }
 

lib/screens/chat/conversations_tab_screen.dart

@@ -64,7 +64,7 @@ class ConversationsTabScreen extends ConsumerWidget {
             );
           }
           return RefreshIndicator(
-            onRefresh: () async => ref.invalidate(conversationsProvider),
+            onRefresh: () => ref.read(conversationsProvider.notifier).refresh(),
             child: ListView.builder(
               itemCount: convs.length,
               itemBuilder: (ctx, i) {

lib/screens/library/project_tasks_screen.dart

@@ -151,8 +151,10 @@ class _ProjectTasksScreenState extends ConsumerState<ProjectTasksScreen> {
     return RefreshIndicator(
       onRefresh: () async {
         setState(() => _pendingStatus.clear());
-        ref.invalidate(projectTasksProvider(widget.projectId));
-        ref.invalidate(projectMilestonesProvider(widget.projectId));
+        await Future.wait([
+          ref.refresh(projectTasksProvider(widget.projectId).future),
+          ref.refresh(projectMilestonesProvider(widget.projectId).future),
+        ]);
       },
       child: CustomScrollView(
         slivers: [

lib/screens/news/news_screen.dart

@@ -97,7 +97,7 @@ class _NewsScreenState extends ConsumerState<NewsScreen> {
             const Divider(height: 1),
             Expanded(
               child: RefreshIndicator(
-                onRefresh: () async => ref.invalidate(newsProvider),
+                onRefresh: () => ref.read(newsProvider.notifier).refresh(),
                 child: ListView.builder(
                 padding:
                     const EdgeInsets.symmetric(horizontal: 8, vertical: 8),

lib/screens/projects/projects_screen.dart

@@ -19,7 +19,7 @@ class ProjectsScreen extends ConsumerWidget {
         data: (projects) => projects.isEmpty
             ? const Center(child: Text('No projects yet.'))
             : RefreshIndicator(
-                onRefresh: () async => ref.invalidate(projectsProvider),
+                onRefresh: () => ref.read(projectsProvider.notifier).refresh(),
                 child: ListView.separated(
                   itemCount: projects.length,
                   separatorBuilder: (_, _) => const Divider(height: 1),