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/photo_pick.dart

@@ -0,0 +1,46 @@
+import 'dart:typed_data';
+
+import 'package:flutter/material.dart';
+import 'package:image_picker/image_picker.dart';
+
+import '../i18n/strings.g.dart';
+
+/// Asks the user to take a photo or pick one from the gallery, then returns its
+/// bytes (or null if cancelled/unavailable). Camera failures — e.g. on desktop,
+/// which has no camera — are swallowed and yield null.
+Future<Uint8List?> pickPhoto(BuildContext context) async {
+  final t = context.t;
+  final source = await showModalBottomSheet<ImageSource>(
+    context: context,
+    builder: (sheetContext) => SafeArea(
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          ListTile(
+            key: const Key('photo.source.camera'),
+            leading: const Icon(Icons.photo_camera_outlined),
+            title: Text(t.photo.camera),
+            onTap: () => Navigator.of(sheetContext).pop(ImageSource.camera),
+          ),
+          ListTile(
+            key: const Key('photo.source.gallery'),
+            leading: const Icon(Icons.photo_library_outlined),
+            title: Text(t.photo.gallery),
+            onTap: () => Navigator.of(sheetContext).pop(ImageSource.gallery),
+          ),
+        ],
+      ),
+    ),
+  );
+  if (source == null) return null;
+  try {
+    final file = await ImagePicker().pickImage(
+      source: source,
+      maxWidth: 1280,
+      imageQuality: 80,
+    );
+    return file?.readAsBytes();
+  } on Object {
+    return null;
+  }
+}

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),

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);