refactor(share): share the widget capture helper

The lyrics card and the rewind slide both rendered a RepaintBoundary to a
temp PNG. captureBoundaryToPngFile does that; pixel ratio, file name, frame
waiting and share parameters stay with each caller.

lib/screens/listening_rewind_screen.dart

@@ -1,17 +1,14 @@
 import 'dart:async';
-import 'dart:io';
 import 'dart:math' as math;
-import 'dart:ui' show ImageByteFormat, ImageFilter;
+import 'dart:ui' show ImageFilter;
 
 import 'package:flutter/material.dart';
-import 'package:flutter/rendering.dart' show RenderRepaintBoundary;
 import 'package:flutter/semantics.dart' show CustomSemanticsAction;
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:intl/intl.dart';
 import 'package:material_3_expressive/material_3_expressive.dart'
     show M3EShapeKind;
-import 'package:path_provider/path_provider.dart';
 import 'package:share_plus/share_plus.dart' show ShareParams, SharePlus, XFile;
 import 'package:spotiflac_android/l10n/l10n.dart';
 import 'package:spotiflac_android/providers/listening_rewind_provider.dart';
@@ -19,6 +16,7 @@ import 'package:spotiflac_android/services/listening_rewind.dart';
 import 'package:spotiflac_android/theme/app_motion.dart';
 import 'package:spotiflac_android/theme/app_tokens.dart';
 import 'package:spotiflac_android/theme/mornye_theme.dart';
+import 'package:spotiflac_android/utils/widget_capture.dart';
 import 'package:spotiflac_android/widgets/app_action_button.dart';
 import 'package:spotiflac_android/widgets/app_bottom_sheet.dart';
 import 'package:spotiflac_android/widgets/app_content_card.dart';

lib/screens/listening_rewind_story.dart

@@ -137,17 +137,12 @@ class _RewindStoryState extends State<_RewindStory>
     final pixelRatio = MediaQuery.devicePixelRatioOf(context);
     setState(() => _sharing = true);
     try {
-      final boundary =
-          _captureKey.currentContext?.findRenderObject()
-              as RenderRepaintBoundary?;
-      if (boundary == null) return;
-      final image = await boundary.toImage(pixelRatio: pixelRatio);
-      final bytes = await image.toByteData(format: ImageByteFormat.png);
-      image.dispose();
-      if (bytes == null) return;
-      final directory = await getTemporaryDirectory();
-      final file = File('${directory.path}/spotiflac_rewind.png');
-      await file.writeAsBytes(bytes.buffer.asUint8List(), flush: true);
+      final file = await captureBoundaryToPngFile(
+        _captureKey,
+        pixelRatio: pixelRatio,
+        fileName: 'spotiflac_rewind.png',
+      );
+      if (file == null) return;
       await SharePlus.instance.share(
         ShareParams(
           files: [XFile(file.path, mimeType: 'image/png')],

lib/screens/lyrics_share_screen.dart

@@ -1,17 +1,15 @@
-import 'dart:io';
-import 'dart:ui' show ImageByteFormat, ImageFilter;
+import 'dart:ui' show ImageFilter;
 
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter/material.dart';
-import 'package:flutter/rendering.dart' show RenderRepaintBoundary;
 import 'package:material_3_expressive/material_3_expressive.dart'
     show M3EShapeKind;
-import 'package:path_provider/path_provider.dart';
 import 'package:share_plus/share_plus.dart' show ShareParams, SharePlus, XFile;
 import 'package:spotiflac_android/l10n/l10n.dart';
 import 'package:spotiflac_android/theme/app_tokens.dart';
 import 'package:spotiflac_android/theme/cover_palette.dart';
 import 'package:spotiflac_android/utils/lyrics_parser.dart';
+import 'package:spotiflac_android/utils/widget_capture.dart';
 import 'package:spotiflac_android/widgets/app_icon.dart';
 import 'package:spotiflac_android/widgets/app_icon_button.dart';
 import 'package:spotiflac_android/widgets/app_shape.dart';
@@ -151,16 +149,12 @@ class _LyricsShareScreenState extends State<LyricsShareScreen> {
       // Share is tapped. Capture the current card after that frame finishes.
       await WidgetsBinding.instance.endOfFrame;
       if (!mounted) return;
-      final boundary =
-          _cardKey.currentContext?.findRenderObject() as RenderRepaintBoundary?;
-      if (boundary == null) throw StateError('Card is not laid out');
-      final image = await boundary.toImage(pixelRatio: 3);
-      final bytes = await image.toByteData(format: ImageByteFormat.png);
-      image.dispose();
-      if (bytes == null) throw StateError('Card could not be encoded');
-      final directory = await getTemporaryDirectory();
-      final file = File('${directory.path}/spotiflac_lyrics.png');
-      await file.writeAsBytes(bytes.buffer.asUint8List(), flush: true);
+      final file = await captureBoundaryToPngFile(
+        _cardKey,
+        pixelRatio: 3,
+        fileName: 'spotiflac_lyrics.png',
+      );
+      if (file == null) throw StateError('Card could not be captured');
       await SharePlus.instance.share(
         ShareParams(
           files: [XFile(file.path, mimeType: 'image/png')],

lib/utils/widget_capture.dart

@@ -0,0 +1,29 @@
+import 'dart:io';
+import 'dart:ui' show ImageByteFormat;
+
+import 'package:flutter/rendering.dart' show RenderRepaintBoundary;
+import 'package:flutter/widgets.dart';
+import 'package:path_provider/path_provider.dart';
+
+/// Renders the [RepaintBoundary] behind [boundaryKey] to a PNG named
+/// [fileName] in the temporary directory.
+///
+/// Returns null when the boundary is not laid out or the image cannot be
+/// encoded, so each caller decides how to report that.
+Future<File?> captureBoundaryToPngFile(
+  GlobalKey boundaryKey, {
+  required double pixelRatio,
+  required String fileName,
+}) async {
+  final boundary =
+      boundaryKey.currentContext?.findRenderObject() as RenderRepaintBoundary?;
+  if (boundary == null) return null;
+  final image = await boundary.toImage(pixelRatio: pixelRatio);
+  final bytes = await image.toByteData(format: ImageByteFormat.png);
+  image.dispose();
+  if (bytes == null) return null;
+  final directory = await getTemporaryDirectory();
+  final file = File('${directory.path}/$fileName');
+  await file.writeAsBytes(bytes.buffer.asUint8List(), flush: true);
+  return file;
+}