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.

pubspec.lock

@@ -201,6 +201,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "1.2.1"
+  code_builder:
+    dependency: transitive
+    description:
+      name: code_builder
+      sha256: "6a6cab2ba4680d6423f34a9b972a4c9a94ebe1b62ecec4e1a1f2cba91fd1319d"
+      url: "https://pub.dev"
+    source: hosted
+    version: "4.11.1"
   collection:
     dependency: transitive
     description:
@@ -996,6 +1004,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "7.0.2"
+  pigeon:
+    dependency: transitive
+    description:
+      name: pigeon
+      sha256: "04cfefc8add8b47ddf9ccac8b92bb4edeb67c87f185c623ba0db118ac99334ad"
+      url: "https://pub.dev"
+    source: hosted
+    version: "26.3.4"
   platform:
     dependency: transitive
     description:
@@ -1481,6 +1497,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.1.3"
+  zxing_barcode_scanner:
+    dependency: transitive
+    description:
+      name: zxing_barcode_scanner
+      sha256: "10f77da15624b1222d1e4b792991bf485b0b7c08d170b99e9a658c69a5fc0a3a"
+      url: "https://pub.dev"
+    source: hosted
+    version: "1.0.4"
 sdks:
   dart: ">=3.11.5 <4.0.0"
   flutter: ">=3.38.4"