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/providers.dart

@@ -2,6 +2,7 @@ import 'package:fintracker_api/fintracker_api.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/api/api_client.dart';
+import '../../core/cache/cached.dart';
 import 'benchmarks_card.dart' show benchmarkRowsProvider;
 
 /// The reporting unit every portfolio screen is scoped to: `all`, `account:<id>` or
@@ -9,37 +10,42 @@ import 'benchmarks_card.dart' show benchmarkRowsProvider;
 /// and the instrument card — three screens showing different scopes would be a trap.
 final scopeProvider = StateProvider<String>((ref) => 'all');
 
+/// A small cross-screen selector (Портфель, Доходы, Налоги, the goal dialog), not a screen's
+/// own primary read — left un-cached per `docs/ai/offline-cache.md`. The underlying `GET`
+/// still gets served from the shared cache transparently on a network failure; this provider
+/// just does not surface `fetchedAt` for a banner.
 final scopesProvider = FutureProvider.autoDispose<List<ScopeOut>>((ref) async {
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsScopes();
   return r.data ?? const [];
 });
 
-final portfolioSummaryProvider = FutureProvider.autoDispose<SummaryOut>((ref) async {
+/// See `docs/ai/offline-cache.md`.
+final portfolioSummaryProvider = FutureProvider.autoDispose<Cached<SummaryOut>>((ref) async {
   final scope = ref.watch(scopeProvider);
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsSummary(scope: scope);
-  return r.data!;
+  return r.cached;
 });
 
-final holdingsProvider = FutureProvider.autoDispose<List<HoldingOut>>((ref) async {
+final holdingsProvider = FutureProvider.autoDispose<Cached<List<HoldingOut>>>((ref) async {
   final scope = ref.watch(scopeProvider);
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsHoldings(scope: scope);
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
-final portfolioReturnsProvider = FutureProvider.autoDispose<List<ReturnsOut>>((ref) async {
+final portfolioReturnsProvider = FutureProvider.autoDispose<Cached<List<ReturnsOut>>>((ref) async {
   final scope = ref.watch(scopeProvider);
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsReturns(scope: scope);
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
-final allocationProvider = FutureProvider.autoDispose<List<AllocationBucket>>((ref) async {
+final allocationProvider = FutureProvider.autoDispose<Cached<List<AllocationBucket>>>((ref) async {
   final scope = ref.watch(scopeProvider);
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsAllocation(scope: scope);
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
 /// Daily portfolio value for the last year, for the chart on Позиции.
-final valueSeriesProvider = FutureProvider.autoDispose<List<ValueDay>>((ref) async {
+final valueSeriesProvider = FutureProvider.autoDispose<Cached<List<ValueDay>>>((ref) async {
   final scope = ref.watch(scopeProvider);
   final now = DateTime.now();
   final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsValueSeries(
@@ -47,17 +53,17 @@ final valueSeriesProvider = FutureProvider.autoDispose<List<ValueDay>>((ref) asy
         from: now.subtract(const Duration(days: 365)),
         to: now,
       );
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
 final instrumentProvider =
-    FutureProvider.autoDispose.family<InstrumentDetail, int>((ref, instrumentId) async {
+    FutureProvider.autoDispose.family<Cached<InstrumentDetail>, int>((ref, instrumentId) async {
   final scope = ref.watch(scopeProvider);
   final r = await ref
       .watch(apiProvider)
       .getInstrumentsApi()
       .instrumentsGet(instrumentId: instrumentId, scope: scope);
-  return r.data!;
+  return r.cached;
 });
 
 /// Every portfolio provider, refreshed together after a metrics rebuild or a pull-to-refresh.