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/ui/qr_scan.dart

@@ -0,0 +1,120 @@
+import 'dart:io' show Platform;
+
+import 'package:flutter/foundation.dart' show kIsWeb;
+import 'package:flutter/material.dart';
+import 'package:zxing_barcode_scanner/zxing_barcode_scanner.dart';
+
+import '../data/species_repository.dart';
+import '../data/variety_repository.dart';
+import '../i18n/strings.g.dart';
+import '../services/seed_label_scan.dart';
+
+/// Whether this platform can open the camera scanner. Mobile-only for now
+/// (pure-ZXing platform views; no Google Play Services) — elsewhere the scan
+/// button simply isn't offered, same pattern as the OCR capture.
+bool get qrScanSupported => !kIsWeb && (Platform.isAndroid || Platform.isIOS);
+
+/// Opens the full-screen camera scanner and returns the first decoded QR
+/// payload, or null if the person backed out. (Ğ1nkgo's scanner pattern on
+/// the same pure-ZXing stack.)
+Future<String?> scanSeedLabelQr(BuildContext context) {
+  return Navigator.of(context).push<String>(
+    MaterialPageRoute<String>(builder: (_) => const _ScannerScreen()),
+  );
+}
+
+/// Acts on a scanned payload: a known seed opens its record (via
+/// [openVariety]); an unknown seed label asks before adding anything; anything
+/// else says so and moves on. Navigation is injected so the flow is
+/// widget-testable without a camera or a router.
+Future<void> handleScannedPayload(
+  BuildContext context, {
+  required VarietyRepository repository,
+  required SpeciesRepository species,
+  required String payload,
+  required void Function(String varietyId) openVariety,
+}) async {
+  final t = context.t;
+  final messenger = ScaffoldMessenger.of(context);
+  final result = await resolveScannedLabel(repository, payload);
+  if (!context.mounted) return;
+
+  if (result.varietyId case final id?) {
+    openVariety(id);
+    return;
+  }
+  final data = result.data;
+  if (data == null) {
+    messenger.showSnackBar(SnackBar(content: Text(t.scan.notALabel)));
+    return;
+  }
+
+  final add = await showDialog<bool>(
+    context: context,
+    builder: (context) => AlertDialog(
+      key: const Key('scan.addPrompt'),
+      title: Text(t.scan.addTitle),
+      content: Text(t.scan.addBody(label: data.varietyLabel)),
+      actions: [
+        TextButton(
+          key: const Key('scan.cancel'),
+          onPressed: () => Navigator.of(context).pop(false),
+          child: Text(t.common.cancel),
+        ),
+        FilledButton(
+          key: const Key('scan.add'),
+          onPressed: () => Navigator.of(context).pop(true),
+          child: Text(t.scan.add),
+        ),
+      ],
+    ),
+  );
+  if (add != true) return;
+
+  final varietyId = await importScannedLabel(
+    repository: repository,
+    species: species,
+    data: data,
+  );
+  messenger.showSnackBar(SnackBar(content: Text(t.scan.added)));
+  openVariety(varietyId);
+}
+
+class _ScannerScreen extends StatefulWidget {
+  const _ScannerScreen();
+
+  @override
+  State<_ScannerScreen> createState() => _ScannerScreenState();
+}
+
+class _ScannerScreenState extends State<_ScannerScreen> {
+  bool _done = false;
+
+  void _onScan(List<BarcodeResult> results) {
+    if (_done || !mounted || results.isEmpty) return;
+    final text = results.first.text;
+    if (text == null || text.isEmpty) return;
+    _done = true;
+    // A beat so the camera view settles before popping (Ğ1nkgo does the same).
+    Future<void>.delayed(const Duration(milliseconds: 150), () {
+      if (mounted) Navigator.of(context).pop(text);
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final t = context.t;
+    return Scaffold(
+      appBar: AppBar(title: Text(t.scan.title)),
+      body: ZxingBarcodeScanner(
+        onScan: _onScan,
+        onError: (error) => Center(
+          child: Padding(
+            padding: const EdgeInsets.all(24),
+            child: Text(error.message ?? t.common.offline),
+          ),
+        ),
+      ),
+    );
+  }
+}