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/di/injector.dart

@@ -12,6 +12,9 @@ import '../db/database.dart';
 import '../db/encrypted_executor.dart';
 import '../security/secret_store.dart';
 import '../security/secure_key_store.dart';
+import '../services/export_import_service.dart';
+import '../services/file_picker_file_service.dart';
+import '../services/file_service.dart';
 
 /// The app's service locator. Kept to the composition root — widgets get their
 /// repositories from here (or via BlocProvider), never by reaching into it deep
@@ -37,12 +40,22 @@ Future<void> configureDependencies() async {
   final speciesRepository = SpeciesRepository(database, idGen: IdGen());
   await speciesRepository.seedBundled(await loadBundledSpecies());
 
+  final varietyRepository = VarietyRepository(
+    database,
+    idGen: IdGen(),
+    nodeId: nodeId,
+  );
+
+  const fileService = FilePickerFileService();
+
   getIt
     ..registerSingleton<SecureKeyStore>(keyStore)
     ..registerSingleton<AppDatabase>(database)
     ..registerSingleton<SpeciesRepository>(speciesRepository)
-    ..registerSingleton<VarietyRepository>(
-      VarietyRepository(database, idGen: IdGen(), nodeId: nodeId),
+    ..registerSingleton<VarietyRepository>(varietyRepository)
+    ..registerSingleton<FileService>(fileService)
+    ..registerSingleton<ExportImportService>(
+      ExportImportService(repository: varietyRepository, files: fileService),
     );
 }