feat(app): offline-кэш на остальных экранах — фаза 5

accounts, cashflow, categories, goals, income, portfolio (+instrument),
rebalance, tax, rules переведены на Cached<T> по контракту
docs/ai/offline-cache.md. Общие/второстепенные селекторы (scopesProvider,
categoriesListProvider и т.п.) оставлены как есть — не основной контент
экрана. sync_page.dart и settings_page.dart не тронуты (первый — заменён
health-page отдельно, второй — чистые действия без списка для баннера).

app/lib/features/portfolio/portfolio_page.dart

@@ -2,8 +2,11 @@ import 'package:fintracker_api/fintracker_api.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../core/cache/cached.dart';
 import '../../core/widgets/async_value_view.dart';
+import '../../core/widgets/stale_banner.dart';
 import 'allocation_tab.dart';
+import 'benchmarks_card.dart';
 import 'holdings_tab.dart';
 import 'providers.dart';
 
@@ -12,11 +15,23 @@ import 'providers.dart';
 /// They are tabs rather than two navigation destinations because they answer two halves of
 /// the same question, and because a tenth item in the bottom bar would leave 40 px per
 /// label on a phone.
+///
+/// One offline banner covers both tabs (`docs/ai/offline-cache.md`) — `TabBarView` builds
+/// both eagerly anyway, so watching every provider here to compute it costs nothing extra.
 class PortfolioPage extends ConsumerWidget {
   const PortfolioPage({super.key});
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final stale = oldestFetch([
+      ref.watch(portfolioSummaryProvider).valueOrNull?.fetchedAt,
+      ref.watch(holdingsProvider).valueOrNull?.fetchedAt,
+      ref.watch(valueSeriesProvider).valueOrNull?.fetchedAt,
+      ref.watch(portfolioReturnsProvider).valueOrNull?.fetchedAt,
+      ref.watch(benchmarkRowsProvider).valueOrNull?.fetchedAt,
+      ref.watch(allocationProvider).valueOrNull?.fetchedAt,
+    ]);
+
     return DefaultTabController(
       length: 2,
       child: Scaffold(
@@ -27,7 +42,18 @@ class PortfolioPage extends ConsumerWidget {
             tabs: [Tab(text: 'Позиции'), Tab(text: 'Аллокация')],
           ),
         ),
-        body: const TabBarView(children: [HoldingsTab(), AllocationTab()]),
+        body: Column(
+          children: [
+            if (stale != null)
+              Padding(
+                padding: const EdgeInsets.fromLTRB(16, 12, 16, 0),
+                child: StaleBanner(fetchedAt: stale),
+              ),
+            const Expanded(
+              child: TabBarView(children: [HoldingsTab(), AllocationTab()]),
+            ),
+          ],
+        ),
       ),
     );
   }