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/lib/ui/backup_section.dart

@@ -0,0 +1,102 @@
+import 'package:flutter/material.dart';
+
+import '../di/injector.dart';
+import '../i18n/strings.g.dart';
+import '../services/export_import_service.dart';
+
+/// The three backup actions shown in Settings: export CSV, export JSON and
+/// import JSON. Import asks for confirmation first (it merges into the live
+/// inventory), then reports what happened in a SnackBar.
+class BackupSection extends StatelessWidget {
+  const BackupSection({this.service, super.key});
+
+  /// Injectable for widget tests; the app resolves it lazily (on tap) from
+  /// the service locator so merely mounting Settings needs no DI setup.
+  final ExportImportService? service;
+
+  ExportImportService get _service => service ?? getIt<ExportImportService>();
+
+  @override
+  Widget build(BuildContext context) {
+    final t = context.t;
+    return Column(
+      children: [
+        ListTile(
+          leading: const Icon(Icons.table_view_outlined),
+          title: Text(t.backup.exportCsv),
+          subtitle: Text(t.backup.exportCsvSubtitle),
+          onTap: () => _runExport(context, () => _service.exportCsv()),
+        ),
+        ListTile(
+          leading: const Icon(Icons.file_download_outlined),
+          title: Text(t.backup.exportJson),
+          subtitle: Text(t.backup.exportJsonSubtitle),
+          onTap: () => _runExport(context, () => _service.exportJson()),
+        ),
+        ListTile(
+          leading: const Icon(Icons.file_upload_outlined),
+          title: Text(t.backup.importJson),
+          subtitle: Text(t.backup.importJsonSubtitle),
+          onTap: () => _runImport(context),
+        ),
+      ],
+    );
+  }
+
+  Future<void> _runExport(
+    BuildContext context,
+    Future<bool> Function() export,
+  ) async {
+    final t = context.t;
+    final messenger = ScaffoldMessenger.of(context);
+    try {
+      final saved = await export();
+      _show(messenger, saved ? t.backup.exportSaved : t.backup.cancelled);
+    } on Object {
+      _show(messenger, t.backup.failed);
+    }
+  }
+
+  Future<void> _runImport(BuildContext context) async {
+    final t = context.t;
+    final messenger = ScaffoldMessenger.of(context);
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (dialogContext) => AlertDialog(
+        title: Text(t.backup.importConfirmTitle),
+        content: Text(t.backup.importConfirmBody),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(dialogContext).pop(false),
+            child: Text(t.common.cancel),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.of(dialogContext).pop(true),
+            child: Text(t.backup.importAction),
+          ),
+        ],
+      ),
+    );
+    if (confirmed != true) return;
+    try {
+      final summary = await _service.importJson();
+      _show(
+        messenger,
+        summary == null
+            ? t.backup.cancelled
+            : t.backup.importDone(
+                added: summary.inserted,
+                updated: summary.updated,
+              ),
+      );
+    } on FormatException {
+      _show(messenger, t.backup.importFailed);
+    } on Object {
+      _show(messenger, t.backup.failed);
+    }
+  }
+
+  void _show(ScaffoldMessengerState messenger, String message) {
+    messenger.showSnackBar(SnackBar(content: Text(message)));
+  }
+}

apps/app_seeds/lib/ui/settings_screen.dart

@@ -2,11 +2,17 @@ import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 
 import '../i18n/strings.g.dart';
+import '../services/export_import_service.dart';
+import 'backup_section.dart';
 import 'theme.dart';
 
-/// Basic settings: app language and an "about" section.
+/// Basic settings: app language, backup (export/import) and an "about"
+/// section. [exportImport] is injectable so widget tests can pass a fake;
+/// the app resolves it lazily from the service locator.
 class SettingsScreen extends StatelessWidget {
-  const SettingsScreen({super.key});
+  const SettingsScreen({this.exportImport, super.key});
+
+  final ExportImportService? exportImport;
 
   @override
   Widget build(BuildContext context) {
@@ -33,6 +39,9 @@ class SettingsScreen extends StatelessWidget {
             onTap: () => LocaleSettings.useDeviceLocale(),
           ),
           const Divider(),
+          _SectionHeader(t.backup.title),
+          BackupSection(service: exportImport),
+          const Divider(),
           _SectionHeader(t.settings.about),
           ListTile(
             leading: Image.asset('assets/logo.png', width: 32),