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.

pubspec.lock

@@ -233,6 +233,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.7"
+  dbus:
+    dependency: transitive
+    description:
+      name: dbus
+      sha256: "0ce9b0a839e6dee59a37a623d2fc26a35bbbe6404213e419b0d6411023d62645"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.7.14"
   drift:
     dependency: transitive
     description:
@@ -281,6 +289,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.1"
+  file_picker:
+    dependency: transitive
+    description:
+      name: file_picker
+      sha256: "57d9a1dd5063f85fa3107fb42d1faffda52fdc948cefd5fe5ea85267a5fc7343"
+      url: "https://pub.dev"
+    source: hosted
+    version: "10.3.10"
   file_selector_linux:
     dependency: transitive
     description: