added support for multiple accounts, guardian login bugfixes, ui changes

lib/widget/child_switcher.dart

@@ -10,6 +10,7 @@ import 'async_action_button.dart';
 import 'centered_leading.dart';
 import 'details_bottom_sheet.dart';
 import 'placeholder_view.dart';
+import 'user_avatar.dart';
 
 /// AppBar action that shows the selected child and lets a guardian switch to
 /// another one. Invisible unless there is a choice to make.
@@ -63,7 +64,7 @@ class ChildTile extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) => ListTile(
-    leading: const CenteredLeading(Icon(Icons.face_outlined)),
+    leading: CenteredLeading(ChildAvatar(child: child)),
     title: Text(child.displayName),
     subtitle: child.className.isEmpty
         ? null
@@ -73,6 +74,38 @@ class ChildTile extends StatelessWidget {
   );
 }
 
+/// Profile picture of a linked child, or its initials while the server does
+/// not name the child's account.
+class ChildAvatar extends StatelessWidget {
+  final GuardianChild child;
+  final int size;
+
+  const ChildAvatar({required this.child, this.size = 20, super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final username = child.username;
+    if (username != null && username.isNotEmpty) {
+      return UserAvatar(
+        id: username,
+        size: size,
+        semanticLabel: child.displayName,
+      );
+    }
+    final colors = Theme.of(context).colorScheme;
+    final initials = [child.firstName, child.lastName]
+        .where((part) => part.isNotEmpty)
+        .map((part) => part.characters.first.toUpperCase())
+        .join();
+    return CircleAvatar(
+      radius: size.toDouble(),
+      backgroundColor: colors.secondaryContainer,
+      foregroundColor: colors.onSecondaryContainer,
+      child: Text(initials, style: TextStyle(fontSize: size * 0.7)),
+    );
+  }
+}
+
 /// Shown by per-child modules while a guardian has no linked child. The
 /// children come from `me/capabilities`, so a failed or pending load must not
 /// be presented as "no child assigned" — that sends parents to the secretariat