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/tax/data/tax_api.dart

@@ -11,6 +11,7 @@ library;
 
 import 'package:dio/dio.dart';
 
+import '../../../core/cache/cached.dart';
 import '../../../core/utils/json.dart';
 
 /// Per-account (or total) tax figures for a year. Every number is an **estimate**: the tax
@@ -153,19 +154,22 @@ class TaxApi {
 
   static const _base = '/api/v1/tax';
 
-  Future<TaxSummary> summary({required int year, int? accountId}) async {
+  /// See `docs/ai/offline-cache.md`: this hand-written client returns `Cached<T>` directly
+  /// (there is no generated `Response<T>` for the screen to unwrap `r.cached` from).
+  Future<Cached<TaxSummary>> summary({required int year, int? accountId}) async {
     final r = await _dio.get<Map<String, dynamic>>(
       _base,
       queryParameters: {'year': year, 'account_id': ?accountId},
     );
-    return TaxSummary.fromJson(r.data ?? const {});
+    return Cached(TaxSummary.fromJson(r.data ?? const {}), fetchedAt: r.extra['fetchedAt'] as DateTime?);
   }
 
-  Future<List<TaxLot>> lots({required int year, int? accountId}) async {
+  Future<Cached<List<TaxLot>>> lots({required int year, int? accountId}) async {
     final r = await _dio.get<Map<String, dynamic>>(
       '$_base/lots',
       queryParameters: {'year': year, 'account_id': ?accountId},
     );
-    return asObjects((r.data ?? const {})['lots']).map(TaxLot.fromJson).toList();
+    final lots = asObjects((r.data ?? const {})['lots']).map(TaxLot.fromJson).toList();
+    return Cached(lots, fetchedAt: r.extra['fetchedAt'] as DateTime?);
   }
 }

app/lib/features/tax/lots_tab.dart

@@ -35,7 +35,8 @@ class _TaxLotsTabState extends ConsumerState<TaxLotsTab> {
       child: AsyncValueView(
         value: lots,
         onRetry: () => ref.invalidate(taxLotsProvider),
-        data: (all) {
+        data: (cached) {
+          final all = cached.data;
           final near = all.where((l) => l.nearLdv).toList();
           // a copy: the provider's list must not be reordered under other watchers
           final rows = [...(_onlyNearLdv ? near : all)];

app/lib/features/tax/providers.dart

@@ -1,6 +1,7 @@
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import '../../core/api/api_client.dart';
+import '../../core/cache/cached.dart';
 import 'data/tax_api.dart';
 
 final taxApiProvider = Provider<TaxApi>((ref) => TaxApi(ref.watch(apiProvider).dio));
@@ -10,14 +11,15 @@ final taxYearProvider = StateProvider<int>((ref) => DateTime.now().year);
 /// Optional account filter; null = all accounts.
 final taxAccountProvider = StateProvider<int?>((ref) => null);
 
-final taxSummaryProvider = FutureProvider.autoDispose<TaxSummary>((ref) async {
+/// See `docs/ai/offline-cache.md`.
+final taxSummaryProvider = FutureProvider.autoDispose<Cached<TaxSummary>>((ref) async {
   return ref.watch(taxApiProvider).summary(
         year: ref.watch(taxYearProvider),
         accountId: ref.watch(taxAccountProvider),
       );
 });
 
-final taxLotsProvider = FutureProvider.autoDispose<List<TaxLot>>((ref) async {
+final taxLotsProvider = FutureProvider.autoDispose<Cached<List<TaxLot>>>((ref) async {
   return ref.watch(taxApiProvider).lots(
         year: ref.watch(taxYearProvider),
         accountId: ref.watch(taxAccountProvider),

app/lib/features/tax/summary_tab.dart

@@ -22,7 +22,8 @@ class TaxSummaryTab extends ConsumerWidget {
       child: AsyncValueView(
         value: summary,
         onRetry: () => ref.invalidate(taxSummaryProvider),
-        data: (data) {
+        data: (cached) {
+          final data = cached.data;
           final totals = data.totals;
           return ListView(
             padding: const EdgeInsets.all(16),

app/lib/features/tax/tax_page.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../../core/cache/cached.dart';
+import '../../core/widgets/stale_banner.dart';
 import 'data/tax_api.dart';
 import 'lots_tab.dart';
 import 'providers.dart';
@@ -16,6 +18,11 @@ class TaxPage extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final stale = oldestFetch([
+      ref.watch(taxSummaryProvider).valueOrNull?.fetchedAt,
+      ref.watch(taxLotsProvider).valueOrNull?.fetchedAt,
+    ]);
+
     return DefaultTabController(
       length: 2,
       child: Scaffold(
@@ -33,10 +40,15 @@ class TaxPage extends ConsumerWidget {
             tabs: [Tab(text: 'Сводка за год'), Tab(text: 'Лоты и ЛДВ')],
           ),
         ),
-        body: const Column(
+        body: Column(
           children: [
-            EstimateBanner(),
-            Expanded(child: TabBarView(children: [TaxSummaryTab(), TaxLotsTab()])),
+            const EstimateBanner(),
+            if (stale != null)
+              Padding(
+                padding: const EdgeInsets.fromLTRB(16, 0, 16, 12),
+                child: StaleBanner(fetchedAt: stale),
+              ),
+            const Expanded(child: TabBarView(children: [TaxSummaryTab(), TaxLotsTab()])),
           ],
         ),
       ),
@@ -50,7 +62,7 @@ class EstimateBanner extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final summary = ref.watch(taxSummaryProvider).valueOrNull;
+    final summary = ref.watch(taxSummaryProvider).valueOrNull?.data;
     final scheme = Theme.of(context).colorScheme;
     final text = summary?.disclaimer ?? TaxSummary.defaultDisclaimer;