feat(photos): camera-or-gallery picker + swipeable full-screen viewer

- Adding a photo (quick-add and detail) now offers a Camera / Gallery chooser
  (shared pickPhoto helper); camera errors on desktop degrade gracefully.
- Full-screen viewer: pinch-to-zoom pans only while zoomed, so a plain
  horizontal swipe pages between photos again.

54 tests green; Linux runs.

apps/app_seeds/lib/ui/quick_add_sheet.dart

@@ -1,46 +1,28 @@
-import 'dart:typed_data';
-
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
-import 'package:image_picker/image_picker.dart';
 
 import '../data/variety_repository.dart';
 import '../i18n/strings.g.dart';
 import '../state/quick_add_cubit.dart';
+import 'photo_pick.dart';
 import 'quantity_picker.dart';
 
-/// Picks a photo and returns its bytes (or null if cancelled). Injected so
-/// widget tests can supply a fake instead of the camera plugin.
-typedef PhotoPicker = Future<Uint8List?> Function();
-
 Future<void> showQuickAddSheet(
   BuildContext context, {
   required VarietyRepository repository,
-  PhotoPicker? photoPicker,
 }) {
   return showModalBottomSheet<void>(
     context: context,
     isScrollControlled: true,
     builder: (_) => BlocProvider(
       create: (_) => QuickAddCubit(repository),
-      child: QuickAddSheet(photoPicker: photoPicker ?? _cameraPhotoPicker),
+      child: const QuickAddSheet(),
     ),
   );
 }
 
-Future<Uint8List?> _cameraPhotoPicker() async {
-  final file = await ImagePicker().pickImage(
-    source: ImageSource.camera,
-    maxWidth: 1280,
-    imageQuality: 80,
-  );
-  return file?.readAsBytes();
-}
-
 class QuickAddSheet extends StatelessWidget {
-  const QuickAddSheet({required this.photoPicker, super.key});
-
-  final PhotoPicker photoPicker;
+  const QuickAddSheet({super.key});
 
   @override
   Widget build(BuildContext context) {
@@ -111,7 +93,7 @@ class QuickAddSheet extends StatelessWidget {
                   label: Text(t.quickAdd.more),
                 ),
               ),
-              if (state.expanded) _MoreSection(photoPicker: photoPicker),
+              if (state.expanded) const _MoreSection(),
               const SizedBox(height: 8),
               Row(
                 children: [
@@ -136,9 +118,7 @@ class QuickAddSheet extends StatelessWidget {
 }
 
 class _MoreSection extends StatelessWidget {
-  const _MoreSection({required this.photoPicker});
-
-  final PhotoPicker photoPicker;
+  const _MoreSection();
 
   @override
   Widget build(BuildContext context) {
@@ -162,7 +142,7 @@ class _MoreSection extends StatelessWidget {
           alignment: Alignment.centerLeft,
           child: OutlinedButton.icon(
             onPressed: () async {
-              final bytes = await photoPicker();
+              final bytes = await pickPhoto(context);
               if (bytes != null) cubit.photoPicked(bytes);
             },
             icon: const Icon(Icons.photo_camera_outlined),