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/core/cache/response_cache.dart

@@ -0,0 +1,71 @@
+import 'dart:convert';
+
+import 'package:drift/drift.dart';
+import 'package:drift_flutter/drift_flutter.dart';
+
+part 'response_cache.g.dart';
+
+/// The last successful body of one GET, keyed by endpoint+params exactly as
+/// [CacheInterceptor] builds the key. See `docs/ai/offline-cache.md` for the
+/// whole contract this table is part of.
+class CachedResponses extends Table {
+  TextColumn get requestKey => text()();
+  TextColumn get body => text()();
+  DateTimeColumn get fetchedAt => dateTime()();
+
+  @override
+  Set<Column> get primaryKey => {requestKey};
+}
+
+/// A cached body plus when it was fetched — what [CacheInterceptor] needs to
+/// rebuild a synthetic [Response] when the network itself is unreachable.
+///
+/// Not named `CachedResponse`: drift already generates a row class with that
+/// name for the `CachedResponses` table above.
+class CacheEntry {
+  const CacheEntry(this.body, this.fetchedAt);
+  final Object? body;
+  final DateTime fetchedAt;
+}
+
+@DriftDatabase(tables: [CachedResponses])
+class ResponseCacheDatabase extends _$ResponseCacheDatabase {
+  ResponseCacheDatabase() : super(_openConnection());
+  ResponseCacheDatabase.forTesting(super.executor);
+
+  @override
+  int get schemaVersion => 1;
+
+  /// Replaces whatever was cached for [requestKey] — one row per endpoint+params,
+  /// never a history.
+  Future<void> put(String requestKey, Object? body) => into(cachedResponses).insertOnConflictUpdate(
+        CachedResponsesCompanion.insert(
+          requestKey: requestKey,
+          body: jsonEncode(body),
+          fetchedAt: DateTime.now(),
+        ),
+      );
+
+  /// The body last stored for [requestKey], or null if this endpoint+params was
+  /// never fetched successfully on this device.
+  Future<CacheEntry?> get(String requestKey) async {
+    final row = await (select(
+      cachedResponses,
+    )..where((t) => t.requestKey.equals(requestKey))).getSingleOrNull();
+    if (row == null) return null;
+    return CacheEntry(jsonDecode(row.body), row.fetchedAt);
+  }
+}
+
+/// One sqlite file via native FFI on Android/iOS/desktop; wasm+OPFS (falling
+/// back to IndexedDB) on web — `driftDatabase` from `package:drift_flutter`
+/// picks the right backend, `web/sqlite3.wasm` and `web/drift_worker.js` are
+/// the compiled assets it loads there (regenerate with `just app-web-assets`
+/// whenever the `drift`/`sqlite3` package versions change).
+QueryExecutor _openConnection() => driftDatabase(
+      name: 'response_cache',
+      web: DriftWebOptions(
+        sqlite3Wasm: Uri.parse('sqlite3.wasm'),
+        driftWorker: Uri.parse('drift_worker.js'),
+      ),
+    );