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;
+  }
+}