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/i18n/en.i18n.json

@@ -43,6 +43,23 @@
     "aboutText": "Local-first, encrypted inventory for traditional seeds. AGPL-3.0.",
     "aboutOpen": "About Tanemaki"
   },
+  "backup": {
+    "title": "Backup",
+    "exportCsv": "Export as CSV",
+    "exportCsvSubtitle": "For spreadsheets — no photos",
+    "exportJson": "Export as JSON",
+    "exportJsonSubtitle": "Complete copy, can be imported back",
+    "importJson": "Import from JSON",
+    "importJsonSubtitle": "Merge a saved copy into this inventory",
+    "importConfirmTitle": "Import a saved copy?",
+    "importConfirmBody": "Entries merge with your inventory; when the same entry exists on both sides, the newest version wins. Nothing gets duplicated.",
+    "importAction": "Import",
+    "exportSaved": "Copy saved",
+    "cancelled": "Cancelled",
+    "importDone": "Imported: {added} new, {updated} updated",
+    "importFailed": "This file could not be read as a Tanemaki copy",
+    "failed": "Something went wrong"
+  },
   "about": {
     "title": "About",
     "kanji": "種まき",