feat(inventory): CSV/JSON export and JSON import with LWW reconciliation

Interchange export/import for Phase 1 (data-model §7):

- JSON: canonical, versioned envelope (formatVersion 1) with all sync
  metadata verbatim, photos embedded as base64, tombstones excluded.
  Species are re-resolved on import by scientific name (catalog ids are
  per-install). Reader tolerates unknown fields/enum values (§5.2) and
  rejects newer format versions with a clear error.
- CSV: export-only spreadsheet flatten, one row per lot, RFC 4180
  escaping, never any photo bytes.
- Import merges by UUIDv7 id in one transaction: insert-if-unknown
  preserving original stamps, last-writer-wins by packed HLC for known
  mutable rows, append-only movements; afterwards the local clock
  receiveEvent()s the newest imported stamp so it never runs behind.
- Settings gains a Backup section (export CSV/JSON, import JSON with
  confirmation); file dialogs behind a FileService interface backed by
  file_picker (MIT).
- Tests: codec round-trip and tolerance, reconciler LWW, repository
  export→import round-trip (fresh DB, idempotent re-import, newer-local
  wins, clock monotonicity, species re-resolution), backup widget flows.

apps/app_seeds/test/ui/backup_section_test.dart

@@ -0,0 +1,150 @@
+import 'dart:convert';
+import 'dart:typed_data';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_localizations/flutter_localizations.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/data/export_import/inventory_json_codec.dart';
+import 'package:tane/data/export_import/inventory_snapshot.dart';
+import 'package:tane/db/database.dart';
+import 'package:tane/i18n/strings.g.dart';
+import 'package:tane/services/export_import_service.dart';
+import 'package:tane/services/file_service.dart';
+import 'package:tane/ui/backup_section.dart';
+import 'package:tane/ui/settings_screen.dart';
+
+import '../support/test_support.dart';
+
+/// Records calls instead of opening real dialogs.
+class FakeFileService implements FileService {
+  String? savedName;
+  Uint8List? savedBytes;
+  Uint8List? bytesToPick;
+
+  @override
+  Future<String?> saveFile({
+    required String suggestedName,
+    required Uint8List bytes,
+  }) async {
+    savedName = suggestedName;
+    savedBytes = bytes;
+    return '/picked/$suggestedName';
+  }
+
+  @override
+  Future<Uint8List?> pickFileBytes({List<String>? allowedExtensions}) async =>
+      bytesToPick;
+}
+
+void main() {
+  late AppDatabase db;
+  late FakeFileService files;
+  late ExportImportService service;
+
+  setUp(() {
+    db = newTestDatabase();
+    files = FakeFileService();
+    service = ExportImportService(
+      repository: newTestRepository(db),
+      files: files,
+    );
+  });
+  tearDown(() => db.close());
+
+  Widget wrap(Widget child) {
+    LocaleSettings.setLocaleSync(AppLocale.en);
+    return TranslationProvider(
+      child: MaterialApp(
+        supportedLocales: AppLocaleUtils.supportedLocales,
+        localizationsDelegates: const [
+          GlobalMaterialLocalizations.delegate,
+          GlobalWidgetsLocalizations.delegate,
+          GlobalCupertinoLocalizations.delegate,
+        ],
+        home: child,
+      ),
+    );
+  }
+
+  testWidgets('settings shows the backup section with its three actions', (
+    tester,
+  ) async {
+    await tester.binding.setSurfaceSize(const Size(800, 1400));
+    addTearDown(() => tester.binding.setSurfaceSize(null));
+    await tester.pumpWidget(wrap(SettingsScreen(exportImport: service)));
+
+    expect(find.text('Backup'), findsOneWidget);
+    expect(find.text('Export as CSV'), findsOneWidget);
+    expect(find.text('Export as JSON'), findsOneWidget);
+    expect(find.text('Import from JSON'), findsOneWidget);
+  });
+
+  testWidgets('export CSV saves a .csv file and confirms', (tester) async {
+    await tester.pumpWidget(
+      wrap(Scaffold(body: BackupSection(service: service))),
+    );
+    await tester.tap(find.text('Export as CSV'));
+    await tester.pumpAndSettle();
+
+    expect(files.savedName, endsWith('.csv'));
+    expect(utf8.decode(files.savedBytes!), startsWith('variety_id,'));
+    expect(find.text('Copy saved'), findsOneWidget);
+  });
+
+  testWidgets('export JSON saves a versioned .json file', (tester) async {
+    await tester.pumpWidget(
+      wrap(Scaffold(body: BackupSection(service: service))),
+    );
+    await tester.tap(find.text('Export as JSON'));
+    await tester.pumpAndSettle();
+
+    expect(files.savedName, endsWith('.json'));
+    final root =
+        jsonDecode(utf8.decode(files.savedBytes!)) as Map<String, dynamic>;
+    expect(root['formatVersion'], inventoryFormatVersion);
+    expect(find.text('Copy saved'), findsOneWidget);
+  });
+
+  testWidgets('import asks for confirmation, imports and reports counts', (
+    tester,
+  ) async {
+    // A valid one-variety export produced by another repository.
+    final otherDb = newTestDatabase();
+    addTearDown(otherDb.close);
+    final other = newTestRepository(otherDb, nodeId: 'node-other');
+    await other.addQuickVariety(label: 'Imported bean');
+    files.bytesToPick = utf8.encode(
+      const InventoryJsonCodec().encode(await other.exportInventory()),
+    );
+
+    await tester.pumpWidget(
+      wrap(Scaffold(body: BackupSection(service: service))),
+    );
+    await tester.tap(find.text('Import from JSON'));
+    await tester.pumpAndSettle();
+    expect(find.text('Import a saved copy?'), findsOneWidget);
+
+    await tester.tap(find.text('Import'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Imported: 1 new, 0 updated'), findsOneWidget);
+    final varieties = await db.select(db.varieties).get();
+    expect(varieties.single.label, 'Imported bean');
+  });
+
+  testWidgets('an unreadable file reports a friendly error', (tester) async {
+    files.bytesToPick = utf8.encode('this is not json');
+    await tester.pumpWidget(
+      wrap(Scaffold(body: BackupSection(service: service))),
+    );
+    await tester.tap(find.text('Import from JSON'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Import'));
+    await tester.pumpAndSettle();
+
+    expect(
+      find.text('This file could not be read as a Tanemaki copy'),
+      findsOneWidget,
+    );
+  });
+}