feat(block2): QR of your npub in the profile (scan to share at a fair)

Adds a QrView (painted from the pure-Dart 'barcode' matrix — already transitive
via pdf, no native plugin) and shows a QR of your npub on the profile identity
card, so you can hand your identity over with a scan. Analyzer clean.

apps/app_seeds/lib/ui/profile_screen.dart

@@ -5,6 +5,7 @@ import '../i18n/strings.g.dart';
 import '../services/profile_store.dart';
 import '../services/social_service.dart';
 import '../services/social_settings.dart';
+import 'qr_view.dart';
 import 'theme.dart';
 
 /// Your own profile: the shareable identity (npub) plus a display name and short
@@ -164,7 +165,9 @@ class _IdentityCard extends StatelessWidget {
             t.profile.idHelp,
             style: const TextStyle(color: seedMuted, fontSize: 12),
           ),
-          const SizedBox(height: 10),
+          const SizedBox(height: 14),
+          Center(child: QrView(data: npub, size: 200)),
+          const SizedBox(height: 12),
           SelectableText(
             npub,
             style: const TextStyle(

apps/app_seeds/lib/ui/qr_view.dart

@@ -0,0 +1,54 @@
+import 'package:barcode/barcode.dart';
+import 'package:flutter/material.dart';
+
+/// Renders [data] as a QR code, painted from the pure-Dart `barcode` matrix (no
+/// native plugin). Used for the profile's shareable identity — hand your npub to
+/// someone at a fair with a scan.
+class QrView extends StatelessWidget {
+  const QrView({required this.data, this.size = 200, super.key});
+
+  final String data;
+  final double size;
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      width: size,
+      height: size,
+      color: Colors.white,
+      padding: const EdgeInsets.all(8),
+      child: CustomPaint(
+        size: Size.square(size),
+        painter: _QrPainter(data),
+      ),
+    );
+  }
+}
+
+class _QrPainter extends CustomPainter {
+  _QrPainter(this.data);
+
+  final String data;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    final black = Paint()..color = Colors.black;
+    for (final element in Barcode.qrCode()
+        .make(data, width: size.width, height: size.height)) {
+      if (element is BarcodeBar && element.black) {
+        canvas.drawRect(
+          Rect.fromLTWH(
+            element.left,
+            element.top,
+            element.width,
+            element.height,
+          ),
+          black,
+        );
+      }
+    }
+  }
+
+  @override
+  bool shouldRepaint(_QrPainter oldDelegate) => oldDelegate.data != data;
+}

apps/app_seeds/pubspec.yaml

@@ -57,6 +57,9 @@ dependencies:
   # Pure-Dart PDF generation (Apache-2.0) for the printable "what I share"
   # catalog. No platform channels, so the whole flow is host-testable.
   pdf: ^3.11.3
+  # Pure-Dart barcode/QR generation (Apache-2.0; already used transitively by
+  # pdf). Painted on screen for the profile's shareable identity QR.
+  barcode: ^2.2.9
   path: ^1.9.0
   path_provider: ^2.1.5