feat(backup): sealed backups + printable recovery sheet

'Save a backup' now writes a sealed .tanemaki file (AES-256-GCM under a
key HKDF-derived from the root seed; format TANEBK v1, open and stable).
Restoring on the same identity is silent; a copy sealed by another
identity asks for the printed recovery code (TANE1 base32, typo-tolerant)
and adopts that seed — recovering your bank recovers you. Legacy plain
exports still restore. Settings gains a recovery sheet (QR + code PDF).
BackupBox/RecoveryCode live in commons_core, pure Dart, TDD.

apps/app_seeds/lib/services/recovery_sheet_service.dart

@@ -0,0 +1,82 @@
+import 'dart:typed_data';
+
+import 'package:flutter/services.dart' show rootBundle;
+import 'package:pdf/pdf.dart';
+import 'package:pdf/widgets.dart' as pw;
+
+import 'file_service.dart';
+
+/// Renders the printable recovery sheet — a QR plus the typed code and short
+/// instructions ("keep two paper copies, like your best seed"). All strings
+/// arrive already localized.
+class RecoverySheetService {
+  RecoverySheetService({required FileService files}) : _files = files;
+
+  final FileService _files;
+
+  Future<Uint8List> buildPdf({
+    required String title,
+    required String intro,
+    required String code,
+  }) async {
+    final base = pw.Font.ttf(
+      await rootBundle.load('assets/fonts/DejaVuSans.ttf'),
+    );
+    final bold = pw.Font.ttf(
+      await rootBundle.load('assets/fonts/DejaVuSans-Bold.ttf'),
+    );
+    final doc = pw.Document(
+      theme: pw.ThemeData.withFont(base: base, bold: bold),
+    );
+    doc.addPage(
+      pw.Page(
+        pageFormat: PdfPageFormat.a4,
+        build: (context) => pw.Column(
+          crossAxisAlignment: pw.CrossAxisAlignment.start,
+          children: [
+            pw.Text(title, style: pw.TextStyle(font: bold, fontSize: 20)),
+            pw.SizedBox(height: 12),
+            pw.Text(intro, style: const pw.TextStyle(fontSize: 11)),
+            pw.SizedBox(height: 24),
+            pw.Center(
+              child: pw.BarcodeWidget(
+                barcode: pw.Barcode.qrCode(),
+                data: code,
+                width: 220,
+                height: 220,
+                // The code is printed below with the Unicode-capable face;
+                // the widget's built-in caption would fall back to Courier.
+                drawText: false,
+              ),
+            ),
+            pw.SizedBox(height: 24),
+            pw.Center(
+              child: pw.Text(
+                code,
+                textAlign: pw.TextAlign.center,
+                style: pw.TextStyle(font: bold, fontSize: 12),
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+    return doc.save();
+  }
+
+  /// Builds the sheet and asks the user where to keep it. Returns true when
+  /// saved, false when they cancelled.
+  Future<bool> saveSheet({
+    required String title,
+    required String intro,
+    required String code,
+    required String suggestedName,
+  }) async {
+    final bytes = await buildPdf(title: title, intro: intro, code: code);
+    final path = await _files.saveFile(
+      suggestedName: suggestedName,
+      bytes: bytes,
+    );
+    return path != null;
+  }
+}