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/variety_detail_screen.dart

@@ -1,6 +1,7 @@
+import 'dart:typed_data';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_bloc/flutter_bloc.dart';
-import 'package:image_picker/image_picker.dart';
 import 'package:material_symbols_icons/symbols.dart';
 import 'package:url_launcher/url_launcher.dart';
 
@@ -10,6 +11,7 @@ import '../db/enums.dart';
 import '../i18n/strings.g.dart';
 import '../state/variety_detail_cubit.dart';
 import 'harvest_date_picker.dart';
+import 'photo_pick.dart';
 import 'quantity_kind_l10n.dart';
 import 'quantity_picker.dart';
 import 'seed_glyph.dart';
@@ -854,12 +856,7 @@ class _PhotoGalleryState extends State<_PhotoGallery> {
   }
 
   Future<void> _addPhoto(VarietyDetailCubit cubit) async {
-    final file = await ImagePicker().pickImage(
-      source: ImageSource.gallery,
-      maxWidth: 1280,
-      imageQuality: 80,
-    );
-    final bytes = await file?.readAsBytes();
+    final bytes = await pickPhoto(context);
     if (bytes != null) await cubit.addPhoto(bytes);
   }
 
@@ -1042,16 +1039,49 @@ class _PhotoViewer extends StatelessWidget {
       body: PageView.builder(
         controller: PageController(initialPage: initialIndex),
         itemCount: photos.length,
-        itemBuilder: (_, i) => InteractiveViewer(
-          minScale: 1,
-          maxScale: 5,
-          child: Center(child: Image.memory(photos[i].bytes)),
-        ),
+        itemBuilder: (_, i) => _ZoomablePhoto(bytes: photos[i].bytes),
       ),
     );
   }
 }
 
+/// Pinch-to-zoom photo that only pans while zoomed, so a plain horizontal
+/// swipe still pages the surrounding [PageView].
+class _ZoomablePhoto extends StatefulWidget {
+  const _ZoomablePhoto({required this.bytes});
+
+  final Uint8List bytes;
+
+  @override
+  State<_ZoomablePhoto> createState() => _ZoomablePhotoState();
+}
+
+class _ZoomablePhotoState extends State<_ZoomablePhoto> {
+  final _controller = TransformationController();
+  bool _zoomed = false;
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return InteractiveViewer(
+      transformationController: _controller,
+      minScale: 1,
+      maxScale: 5,
+      panEnabled: _zoomed,
+      onInteractionEnd: (_) {
+        final zoomed = _controller.value.getMaxScaleOnAxis() > 1.01;
+        if (zoomed != _zoomed) setState(() => _zoomed = zoomed);
+      },
+      child: Center(child: Image.memory(widget.bytes)),
+    );
+  }
+}
+
 class _SectionTitle extends StatelessWidget {
   const _SectionTitle(this.text);