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_service.dart

@@ -0,0 +1,19 @@
+import 'dart:typed_data';
+
+/// Cross-platform "save a file / open a file" boundary, so everything above
+/// it (services, tests) stays free of platform channels. Implementations must
+/// never leave plaintext temp copies behind — bytes go straight between
+/// memory and the destination the user picked (security-privacy.md).
+abstract class FileService {
+  /// Asks the user where to save [bytes] (suggesting [suggestedName]) and
+  /// writes them there. Returns the chosen path, or null if they cancelled.
+  Future<String?> saveFile({
+    required String suggestedName,
+    required Uint8List bytes,
+  });
+
+  /// Asks the user to pick a file (optionally restricted to
+  /// [allowedExtensions], without dots) and returns its bytes, or null if
+  /// they cancelled.
+  Future<Uint8List?> pickFileBytes({List<String>? allowedExtensions});
+}