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/pubspec.yaml

@@ -60,6 +60,10 @@ dependencies:
   # Pure-Dart barcode/QR generation (Apache-2.0; already used transitively by
   # pdf). Painted on screen for the profile's shareable identity QR.
   barcode: ^2.2.9
+  # Camera QR scanning via pure ZXing (MIT) — no ML Kit, no Google Play
+  # Services (F-Droid-safe; same stack Ğ1nkgo ships). Mobile-only; other
+  # platforms simply don't offer the scan button.
+  zxing_barcode_scanner: ^1.0.3
   path: ^1.9.0
   path_provider: ^2.1.5