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/test/ui/qr_scan_test.dart

@@ -0,0 +1,133 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:tane/data/export_import/seed_label_codec.dart';
+import 'package:tane/data/species_repository.dart';
+import 'package:tane/data/variety_repository.dart';
+import 'package:tane/db/database.dart';
+import 'package:tane/ui/qr_scan.dart';
+
+import '../support/test_support.dart';
+
+/// Hosts a button that feeds [payload] to [handleScannedPayload], standing in
+/// for the camera — the handler is what carries the flow's logic.
+class _ScanHost extends StatelessWidget {
+  const _ScanHost({
+    required this.repository,
+    required this.species,
+    required this.payload,
+    required this.opened,
+  });
+
+  final VarietyRepository repository;
+  final SpeciesRepository species;
+  final String payload;
+  final List<String> opened;
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: Center(
+        child: ElevatedButton(
+          key: const Key('scanHost.fire'),
+          onPressed: () => handleScannedPayload(
+            context,
+            repository: repository,
+            species: species,
+            payload: payload,
+            openVariety: opened.add,
+          ),
+          child: const Text('scan'),
+        ),
+      ),
+    );
+  }
+}
+
+void main() {
+  late AppDatabase db;
+  late VarietyRepository repo;
+  late SpeciesRepository species;
+
+  setUp(() {
+    db = newTestDatabase();
+    repo = newTestRepository(db);
+    species = newTestSpeciesRepository(db);
+  });
+
+  tearDown(() => db.close());
+
+  Future<List<String>> pumpAndFire(WidgetTester tester, String payload) async {
+    final opened = <String>[];
+    await tester.pumpWidget(
+      wrapScreen(
+        repository: repo,
+        child: _ScanHost(
+          repository: repo,
+          species: species,
+          payload: payload,
+          opened: opened,
+        ),
+      ),
+    );
+    await tester.tap(find.byKey(const Key('scanHost.fire')));
+    await tester.pumpAndSettle();
+    return opened;
+  }
+
+  testWidgets('a known label opens its record straight away', (tester) async {
+    final id = await repo.addQuickVariety(label: 'Tomate Rosa');
+    final payload = SeedLabelCodec.encode(
+      const SeedLabelData(varietyLabel: 'tomate rosa'),
+    );
+    final opened = await pumpAndFire(tester, payload);
+    expect(opened, [id]);
+    expect(find.byKey(const Key('scan.addPrompt')), findsNothing);
+    await disposeTree(tester);
+  });
+
+  testWidgets('an unknown label asks first, then adds and opens it', (
+    tester,
+  ) async {
+    final payload = SeedLabelCodec.encode(
+      const SeedLabelData(
+        varietyLabel: 'Acelga de Perales',
+        year: 2024,
+        origin: 'María',
+      ),
+    );
+    final opened = await pumpAndFire(tester, payload);
+    expect(find.byKey(const Key('scan.addPrompt')), findsOneWidget);
+    expect(opened, isEmpty); // nothing created before the person says so
+    expect(await db.select(db.varieties).get(), isEmpty);
+
+    await tester.tap(find.byKey(const Key('scan.add')));
+    await tester.pumpAndSettle();
+
+    final varieties = await db.select(db.varieties).get();
+    expect(varieties.single.label, 'Acelga de Perales');
+    final lots = await db.select(db.lots).get();
+    expect(lots.single.harvestYear, 2024);
+    expect(lots.single.originName, 'María');
+    expect(opened, [varieties.single.id]);
+    await disposeTree(tester);
+  });
+
+  testWidgets('declining the prompt adds nothing', (tester) async {
+    final payload = SeedLabelCodec.encode(
+      const SeedLabelData(varietyLabel: 'Acelga de Perales'),
+    );
+    final opened = await pumpAndFire(tester, payload);
+    await tester.tap(find.byKey(const Key('scan.cancel')));
+    await tester.pumpAndSettle();
+    expect(await db.select(db.varieties).get(), isEmpty);
+    expect(opened, isEmpty);
+    await disposeTree(tester);
+  });
+
+  testWidgets('a non-label QR just says so', (tester) async {
+    final opened = await pumpAndFire(tester, 'https://example.org/whatever');
+    expect(find.text('That code is not a seed label'), findsOneWidget);
+    expect(opened, isEmpty);
+    await disposeTree(tester);
+  });
+}