feat(app): scan a printed seed label back into its record

Closes the physical↔digital loop the labels opened: the QR on a printed
envelope (tane://seed, already encoded by seed_label_codec) can now be
scanned from the inventory bar. A known label opens its variety; an
unknown one asks before adding anything, then creates the variety (species
linked by exact scientific name when bundled) with a lot carrying the
label's year and origin. Scanner is pure ZXing (zxing_barcode_scanner,
MIT) — no ML Kit, no Play Services, F-Droid-safe, the stack Ğ1nkgo ships.
Mobile-only; other platforms don't show the button (OCR precedent).
The post-scan flow is a plain function (handleScannedPayload) so it's
widget-tested without a camera.

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

@@ -368,6 +368,15 @@
   "saved": "Labels saved",
   "cancelled": "Cancelled"
  },
+ "scan": {
+  "action": "Scan a seed label",
+  "title": "Scan a label",
+  "notALabel": "That code is not a seed label",
+  "addTitle": "Not in your collection",
+  "addBody": "Add “{label}” to your seeds?",
+  "add": "Add it",
+  "added": "Added to your collection"
+ },
  "cropCalendar": {
   "add": "Crop calendar",
   "title": "Crop calendar",