feat: Add invoice expiry countdown to lighting qr code

mobile/lib/common/countdown.dart

@@ -0,0 +1,52 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+
+class Countdown extends StatefulWidget {
+  final int start;
+
+  const Countdown({super.key, required this.start});
+
+  @override
+  CountdownState createState() => CountdownState();
+}
+
+class CountdownState extends State<Countdown> {
+  Timer? _timer;
+  int _start = 0;
+
+  @override
+  void initState() {
+    super.initState();
+    _start = widget.start;
+    _timer = Timer.periodic(const Duration(seconds: 1), (timer) {
+      setState(() {
+        if (_start > 0) {
+          _start--;
+        } else {
+          _timer?.cancel();
+        }
+      });
+    });
+  }
+
+  @override
+  void dispose() {
+    _timer?.cancel();
+    super.dispose();
+  }
+
+  String get _formattedTime {
+    final minutes = _start ~/ 60;
+    final seconds = _start % 60;
+    return '${minutes.toString().padLeft(2, '0')}:${seconds.toString().padLeft(2, '0')}';
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(
+      _formattedTime,
+      style: const TextStyle(fontSize: 14),
+    );
+  }
+}

mobile/lib/features/trade/channel_creation_flow/channel_funding_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:get_10101/common/bitcoin_balance_field.dart';
 import 'package:get_10101/common/color.dart';
+import 'package:get_10101/common/countdown.dart';
 import 'package:get_10101/common/custom_qr_code.dart';
 import 'package:get_10101/common/domain/funding_channel_task.dart';
 import 'package:get_10101/common/domain/model.dart';
@@ -60,6 +61,9 @@ class ChannelFunding extends StatefulWidget {
 class _ChannelFunding extends State<ChannelFunding> {
   FundingType selectedBox = FundingType.onchain;
 
+  // TODO(holzeis): It would be nicer if this would come directly from the invoice.
+  final expiry = DateTime.timestamp().second + 300;
+
   @override
   Widget build(BuildContext context) {
     final status = context.watch<FundingChannelChangeNotifier>().status;
@@ -206,12 +210,24 @@ class _ChannelFunding extends State<ChannelFunding> {
                                             "Copied: $qrCodeSubTitle");
                                       });
                                     },
-                                    child: Text(
-                                      truncateWithEllipsis(44, qrCodeSubTitle),
-                                      style: const TextStyle(fontSize: 14),
-                                      textAlign: TextAlign.center,
-                                      maxLines: 1,
-                                      overflow: TextOverflow.ellipsis,
+                                    child: Column(
+                                      children: [
+                                        Text(
+                                          truncateWithEllipsis(44, qrCodeSubTitle),
+                                          style: const TextStyle(fontSize: 14),
+                                          textAlign: TextAlign.center,
+                                          maxLines: 1,
+                                          overflow: TextOverflow.ellipsis,
+                                        ),
+                                        if (selectedBox == FundingType.lightning)
+                                          Padding(
+                                            padding: const EdgeInsets.only(top: 2),
+                                            child: Countdown(
+                                                start: expiry > DateTime.timestamp().second
+                                                    ? expiry - DateTime.timestamp().second
+                                                    : 0),
+                                          ),
+                                      ],
                                     ),
                                   ),
                                 ),