feat(labels): print seed labels with QR from an inventory selection

Add multi-select to the inventory and a "Print labels" flow that renders a
PDF sheet of labels (small stickers or large cards), each carrying the seed's
common name, variety, scientific name, year/origin and a tanemaki://seed QR
that holds the seed's data offline (future scan-to-import).

- SeedLabelCodec: compact, versioned tanemaki://seed URI (encode/decode)
- LabelSheetService: pure-Dart PDF grid (pdf + barcode + DejaVu), RTL-aware
- VarietyRepository.labelRows: one row per lot, locale-picked common name
- InventoryCubit selection mode; contextual app bar + label print sheet
- i18n en/es/pt (printLabels.*)

Tests: codec, PDF service (both formats + RTL), labelRows, selection cubit,
inventory widget selection.

apps/app_seeds/lib/i18n/en.i18n.json

@@ -262,6 +262,23 @@
     "catalogSaved": "Catalog saved",
     "cancelled": "Cancelled"
   },
+  "printLabels": {
+    "action": "Print labels",
+    "title": "Print labels",
+    "selectHint": "Pick the seeds to print labels for",
+    "selectAll": "Select all",
+    "selected": "{n} selected",
+    "none": "Select seeds first",
+    "format": "Label size",
+    "formatStickers": "Small stickers",
+    "formatStickersHint": "Many small labels per page — to peel onto packets",
+    "formatCards": "Large cards",
+    "formatCardsHint": "Fewer, bigger labels — for jars and boxes",
+    "count": "{n} labels",
+    "save": "Save labels",
+    "saved": "Labels saved",
+    "cancelled": "Cancelled"
+  },
   "cropCalendar": {
     "add": "Crop calendar",
     "title": "Crop calendar",