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/services/file_picker_file_service.dart

@@ -0,0 +1,47 @@
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:file_picker/file_picker.dart';
+
+import 'file_service.dart';
+
+/// [FileService] backed by the `file_picker` plugin (one plugin covers the
+/// save and open dialogs on Android, iOS and desktop).
+class FilePickerFileService implements FileService {
+  const FilePickerFileService();
+
+  @override
+  Future<String?> saveFile({
+    required String suggestedName,
+    required Uint8List bytes,
+  }) async {
+    final path = await FilePicker.platform.saveFile(
+      fileName: suggestedName,
+      bytes: bytes,
+    );
+    if (path == null) return null;
+    // On mobile the plugin writes [bytes] itself; on desktop it only returns
+    // the chosen path, so write them here.
+    if (!Platform.isAndroid && !Platform.isIOS) {
+      await File(path).writeAsBytes(bytes, flush: true);
+    }
+    return path;
+  }
+
+  @override
+  Future<Uint8List?> pickFileBytes({List<String>? allowedExtensions}) async {
+    final result = await FilePicker.platform.pickFiles(
+      type: allowedExtensions == null ? FileType.any : FileType.custom,
+      allowedExtensions: allowedExtensions,
+      withData: true,
+    );
+    final file = result?.files.singleOrNull;
+    if (file == null) return null;
+    // withData should populate bytes; fall back to the path just in case.
+    final bytes = file.bytes;
+    if (bytes != null) return bytes;
+    final path = file.path;
+    if (path == null) return null;
+    return File(path).readAsBytes();
+  }
+}