feat(profile): Material default avatar + square photo crop

Drop the DiceBear generated avatar (and dicebear_core/dicebear_styles/
flutter_svg): the default is again a Material coloured-initial disc from
the pubkey. Picking a photo now goes through a square crop step
(crop_your_image — pure Flutter, all platforms incl. desktop, Apache-2.0,
only pulls the image package we already had) before the 24 KB thumbnail.

apps/app_seeds/test/ui/photo_crop_test.dart

@@ -0,0 +1,48 @@
+import 'dart:typed_data';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+import 'package:tane/ui/photo_crop.dart';
+
+void main() {
+  // A small but real PNG the cropper can decode.
+  final bytes = Uint8List.fromList(
+    img.encodePng(img.Image(width: 32, height: 32)),
+  );
+
+  testWidgets('cropToSquare shows a cancel/confirm UI and cancel returns null', (
+    tester,
+  ) async {
+    Uint8List? result;
+    var returned = false;
+    await tester.pumpWidget(MaterialApp(
+      home: Builder(
+        builder: (context) => Scaffold(
+          body: Center(
+            child: ElevatedButton(
+              onPressed: () async {
+                result = await cropToSquare(context, bytes);
+                returned = true;
+              },
+              child: const Text('open'),
+            ),
+          ),
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('open'));
+    await tester.pump(); // push the crop route
+    await tester.pump(const Duration(milliseconds: 100));
+
+    expect(find.byKey(const Key('crop.confirm')), findsOneWidget);
+    expect(find.byKey(const Key('crop.cancel')), findsOneWidget);
+
+    await tester.tap(find.byKey(const Key('crop.cancel')));
+    await tester.pump(); // pop the crop route
+
+    expect(returned, isTrue);
+    expect(result, isNull); // cancelling yields no image
+  });
+}