feat(app): offline-кэш GET-запросов на дашборде — фаза 5

CacheInterceptor кэширует каждый успешный GET по путь+параметры в drift
(sqlite нативно, wasm+OPFS в браузере) и подменяет им сетевую ошибку;
провайдер отдаёт Cached<T>, экран показывает баннер «данные на …».
Контракт для остальных экранов — docs/ai/offline-cache.md.

flake.nix: libsecret/pkg-config для линуксовой сборки, jq/curl для
just app-web-assets (сборка sqlite3.wasm + drift_worker.js).

app/lib/features/home/providers.dart

@@ -3,54 +3,58 @@ 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';
 
-final netWorthBreakdownProvider = FutureProvider.autoDispose<NetWorthBreakdown>((ref) async {
+/// Every provider below returns `Cached<T>`, not `T`: `HomePage` reads
+/// `.data` for the tiles and `.fetchedAt` (via `oldestFetch`) for the one
+/// offline banner at the top. See `docs/ai/offline-cache.md`.
+final netWorthBreakdownProvider = FutureProvider.autoDispose<Cached<NetWorthBreakdown>>((ref) async {
   final r = await ref.watch(apiProvider).getNetworthApi().networthBreakdown();
-  return r.data!;
+  return r.cached;
 });
 
 /// Daily net worth for the last 365 days.
-final netWorthSeriesProvider = FutureProvider.autoDispose<List<NetWorthDay>>((ref) async {
+final netWorthSeriesProvider = FutureProvider.autoDispose<Cached<List<NetWorthDay>>>((ref) async {
   final now = DateTime.now();
   final r = await ref.watch(apiProvider).getNetworthApi().networthSeries(
         from: now.subtract(const Duration(days: 365)),
         to: now,
       );
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
 /// The current (partial) month's cashflow, or null before any data exists.
-final cashflowThisMonthProvider = FutureProvider.autoDispose<CashFlowMonth?>((ref) async {
+final cashflowThisMonthProvider = FutureProvider.autoDispose<Cached<CashFlowMonth?>>((ref) async {
   final r = await ref.watch(apiProvider).getCashflowApi().cashflowMonthly(months: 1);
   final rows = r.data ?? const [];
-  return rows.isEmpty ? null : rows.last;
+  return Cached(rows.isEmpty ? null : rows.last, fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
 /// The last 12 months, for the income-vs-expense bar chart.
-final cashflowLast12Provider = FutureProvider.autoDispose<List<CashFlowMonth>>((ref) async {
+final cashflowLast12Provider = FutureProvider.autoDispose<Cached<List<CashFlowMonth>>>((ref) async {
   final r = await ref.watch(apiProvider).getCashflowApi().cashflowMonthly(months: 12);
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
-final runwayProvider = FutureProvider.autoDispose<RunwayOut>((ref) async {
+final runwayProvider = FutureProvider.autoDispose<Cached<RunwayOut>>((ref) async {
   final r = await ref.watch(apiProvider).getCashflowApi().cashflowRunway();
-  return r.data!;
+  return r.cached;
 });
 
 /// When the metric tables were last rebuilt; null before the first refresh.
-final metricsStatusProvider = FutureProvider.autoDispose<RefreshLogOut?>((ref) async {
+final metricsStatusProvider = FutureProvider.autoDispose<Cached<RefreshLogOut?>>((ref) async {
   try {
     final r = await ref.watch(apiProvider).getMetricsApi().metricsStatus();
-    return r.data;
+    return r.cached;
   } on DioException catch (e) {
-    if (e.response?.statusCode == 404) return null;
+    if (e.response?.statusCode == 404) return const Cached(null);
     rethrow;
   }
 });
 
-final dataQualityProvider = FutureProvider.autoDispose<List<DataQualityRow>>((ref) async {
+final dataQualityProvider = FutureProvider.autoDispose<Cached<List<DataQualityRow>>>((ref) async {
   final r = await ref.watch(apiProvider).getMetricsApi().metricsDataQuality();
-  return r.data ?? const [];
+  return Cached(r.data ?? const [], fetchedAt: r.extra['fetchedAt'] as DateTime?);
 });
 
 /// Every dashboard provider, refreshed together after a manual
@@ -68,12 +72,12 @@ void invalidateHomeProviders(WidgetRef ref) {
 
 /// The investment side of the dashboard: one scope-wide summary, `all` by default.
 /// Null when the ledger is empty, which is the normal state before a broker sync.
-final portfolioSummaryHomeProvider = FutureProvider.autoDispose<SummaryOut?>((ref) async {
+final portfolioSummaryHomeProvider = FutureProvider.autoDispose<Cached<SummaryOut?>>((ref) async {
   try {
     final r = await ref.watch(apiProvider).getAnalyticsApi().analyticsSummary();
-    return r.data;
+    return r.cached;
   } on DioException catch (e) {
-    if (e.response?.statusCode == 404) return null;
+    if (e.response?.statusCode == 404) return const Cached(null);
     rethrow;
   }
 });