implemented comprehensive accessibility (A11y) support across the app

lib/widget/large_profile_picture_view.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:photo_view/photo_view.dart';
 
 import '../model/account_data.dart';
+import 'a11y/a11y_labels.dart';
 import 'user_avatar.dart';
 
 class LargeProfilePictureView extends StatelessWidget {
@@ -15,18 +16,25 @@ class LargeProfilePictureView extends StatelessWidget {
   });
 
   @override
-  Widget build(BuildContext context) => Scaffold(
-    appBar: AppBar(title: Text(isGroup ? 'Gruppenbild' : 'Profilbild')),
-    body: PhotoView(
-      minScale: 0.5,
-      maxScale: 3.0,
-      imageProvider: Image.network(
-        avatarUrl(id: id, isGroup: isGroup, size: 1024),
-        headers: {'Authorization': AccountData().getBasicAuthHeader()},
-      ).image,
-      backgroundDecoration: BoxDecoration(
-        color: Theme.of(context).colorScheme.surface,
+  Widget build(BuildContext context) {
+    final label = isGroup ? A11yLabels.groupPicture : A11yLabels.profilePicture;
+    return Scaffold(
+      appBar: AppBar(title: Text(label)),
+      body: Semantics(
+        image: true,
+        label: label,
+        child: PhotoView(
+          minScale: 0.5,
+          maxScale: 3.0,
+          imageProvider: Image.network(
+            avatarUrl(id: id, isGroup: isGroup, size: 1024),
+            headers: {'Authorization': AccountData().getBasicAuthHeader()},
+          ).image,
+          backgroundDecoration: BoxDecoration(
+            color: Theme.of(context).colorScheme.surface,
+          ),
+        ),
       ),
-    ),
-  );
+    );
+  }
 }