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/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),