fix(app): 海报下载反馈改弹层内呈现——SnackBar 被模态弹层遮挡不可见

根因：保存成功/失败的 SnackBar 弹在弹层底下的页面 Scaffold 上，被模态
bottom sheet + 遮罩整层盖住，用户永远看不到（保存本身大概率成功）。

- 成功：下载按钮翻成功态（✓+已保存到相册，kColorSuccess）2.5s 自动复原
- 失败：按钮下方弹层内红字留置可重试（对齐隐藏照片弹层面板内提示模式）
- 删除被遮挡的 SnackBar 路径；复原 Timer 于 dispose 取消

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/mobile/lib/pages/share_qr_dialog.dart

@@ -8,6 +8,7 @@
 /// 相册未公开时琥珀提示明确「该链接目前无法公开浏览」；URL 文本可长按复制。
 library;
 
+import 'dart:async';
 import 'dart:ui' as ui;
 
 import 'package:dio/dio.dart';
@@ -338,10 +339,13 @@ class _PosterSheet extends ConsumerStatefulWidget {
   ConsumerState<_PosterSheet> createState() => _PosterSheetState();
 }
 
+/// 保存状态机：idle → saving →（success 自动复原 / error 留置可重试）。
+/// 反馈一律在弹层内呈现——模态弹层遮挡页面 SnackBar，页面级提示不可见。
+enum _SaveState { idle, saving, success, error }
+
 class _PosterSheetState extends ConsumerState<_PosterSheet> {
   final GlobalKey _posterKey = GlobalKey();
   String? _bannerUrl;
-  bool _saving = false;
 
   /// 海报语言（'zh'|'en'；打开时刻快照 App 语言前缀，切换只影响海报）。
   late String _posterLang =
@@ -349,6 +353,15 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
           ? 'zh'
           : 'en';
 
+  _SaveState _saveState = _SaveState.idle;
+  Timer? _saveResetTimer;
+
+  @override
+  void dispose() {
+    _saveResetTimer?.cancel();
+    super.dispose();
+  }
+
   @override
   void initState() {
     super.initState();
@@ -366,20 +379,23 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
   }
 
   Future<void> _save() async {
-    setState(() => _saving = true);
+    _saveResetTimer?.cancel();
-    final messenger = ScaffoldMessenger.of(context);
+    setState(() => _saveState = _SaveState.saving);
-    final l10n = AppLocalizations.of(context);
     try {
       final boundary =
           _posterKey.currentContext!.findRenderObject()
               as RenderRepaintBoundary;
       final image = await boundary.toImage(pixelRatio: 3);
       await ref.read(posterSaverProvider)(image);
-      messenger.showSnackBar(SnackBar(content: Text(l10n.posterSaved)));
+      if (!mounted) return;
+      // 成功：按钮翻成功态，2.5s 后自动复原（期间可切语言再存一张）。
+      setState(() => _saveState = _SaveState.success);
+      _saveResetTimer = Timer(const Duration(milliseconds: 2500), () {
+        if (mounted) setState(() => _saveState = _SaveState.idle);
+      });
     } on Object {
-      messenger.showSnackBar(SnackBar(content: Text(l10n.posterSaveFailed)));
+      // 失败：弹层内红字留置、按钮复位可重试（对齐隐藏照片弹层模式）。
-    } finally {
+      if (mounted) setState(() => _saveState = _SaveState.error);
-      if (mounted) setState(() => _saving = false);
     }
   }
 
@@ -563,18 +579,47 @@ class _PosterSheetState extends ConsumerState<_PosterSheet> {
                 ),
               ),
               const SizedBox(height: kSpace16),
-              // ── 下载海报图 ──
+              // ── 下载海报图：成功=按钮翻成功态自动复原；失败=下行红字可重试 ──
               FilledButton.icon(
-                onPressed: _saving ? null : _save,
+                onPressed: _saveState == _SaveState.saving ? null : _save,
-                icon: _saving
+                style: _saveState == _SaveState.success
-                    ? const SizedBox(
+                    ? FilledButton.styleFrom(
+                        backgroundColor:
+                            theme.brightness == Brightness.light
+                                ? kColorSuccess
+                                : kColorSuccessDark,
+                      )
+                    : null,
+                icon: switch (_saveState) {
+                  _SaveState.saving => const SizedBox(
                       width: 16,
                       height: 16,
                       child: CircularProgressIndicator(strokeWidth: 2),
-                      )
-                    : const Icon(Icons.download_outlined, size: 18),
-                label: Text(l10n.posterDownload),
                     ),
+                  _SaveState.success => const Icon(
+                      Icons.check_circle_outline,
+                      size: 18,
+                    ),
+                  _ => const Icon(Icons.download_outlined, size: 18),
+                },
+                label: Text(
+                  _saveState == _SaveState.success
+                      ? l10n.posterSaved
+                      : l10n.posterDownload,
+                ),
+              ),
+              if (_saveState == _SaveState.error) ...[
+                const SizedBox(height: kSpace8),
+                Text(
+                  l10n.posterSaveFailed,
+                  textAlign: TextAlign.center,
+                  style: theme.textTheme.bodySmall?.copyWith(
+                    color: theme.brightness == Brightness.light
+                        ? kColorError
+                        : kColorErrorDark,
+                  ),
+                ),
+              ],
             ],
           ),
         ),

apps/mobile/test/pages/share_qr_page_test.dart

@@ -5,7 +5,9 @@
 /// 主操作写剪贴板 + 已复制提示、Open in browser 次操作经注入替身启动器
 /// 打开并退出页面、未公开相册显示「该链接目前无法公开浏览」琥珀提示
 /// （公开相册不显示）、海报弹层内「中文/English」切换（默认跟 App 语言，
-/// 提示文案/相册标题/二维码 lang 随切换，弹层 chrome 仍跟 App 语言）。
+/// 提示文案/相册标题/二维码 lang 随切换，弹层 chrome 仍跟 App 语言）、
+/// 下载海报反馈在弹层内可见（成功=按钮翻成功态自动复原；失败=弹层内
+/// 红字可重试；SnackBar 在模态弹层下不可见，不得作为反馈通道）。
 library;
 
 import 'package:flutter/material.dart';
@@ -26,12 +28,15 @@ void main() {
     WidgetTester tester, {
     bool publicEnabled = true,
     UrlLauncher? launcher,
+    PosterSaver? saver,
   }) async {
     await tester.pumpWidget(
       ProviderScope(
         overrides: [
           if (launcher != null)
             urlLauncherProvider.overrideWith((ref) => launcher),
+          if (saver != null)
+            posterSaverProvider.overrideWith((ref) => saver),
         ],
         child: MaterialApp(
           localizationsDelegates: AppLocalizations.localizationsDelegates,
@@ -121,6 +126,72 @@ void main() {
     expect(find.text('Download poster image'), findsOneWidget);
   });
 
+  testWidgets('下载海报成功：按钮翻成功态并自动复原，全程无 SnackBar', (
+    tester,
+  ) async {
+    await pumpPage(tester, saver: (_) async {});
+    await tester.tap(find.text('Generate QR poster'));
+    await tester.pumpAndSettle();
+    await tester.ensureVisible(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+
+    // 反馈必须在弹层内可见（SnackBar 被模态弹层遮挡，禁用）。
+    expect(find.byType(SnackBar), findsNothing);
+    expect(
+      find.descendant(
+        of: find.byType(BottomSheet),
+        matching: find.text('Poster saved to gallery'),
+      ),
+      findsOneWidget,
+      reason: '成功反馈显示在下载按钮上（弹层内）',
+    );
+
+    // 约 2.5s 后自动复原，可再次下载（配合语言切换存第二张）。
+    await tester.pump(const Duration(seconds: 3));
+    await tester.pumpAndSettle();
+    expect(find.text('Poster saved to gallery'), findsNothing);
+    expect(find.text('Download poster image'), findsOneWidget);
+  });
+
+  testWidgets('下载海报失败：弹层内红字提示且可重试', (tester) async {
+    var calls = 0;
+    await pumpPage(
+      tester,
+      saver: (_) async {
+        calls++;
+        throw StateError('save failed');
+      },
+    );
+    await tester.tap(find.text('Generate QR poster'));
+    await tester.pumpAndSettle();
+    await tester.ensureVisible(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+
+    await tester.tap(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+
+    expect(find.byType(SnackBar), findsNothing);
+    expect(
+      find.descendant(
+        of: find.byType(BottomSheet),
+        matching: find.text('Failed to save poster'),
+      ),
+      findsOneWidget,
+      reason: '失败反馈在弹层内（对齐隐藏照片弹层的面板内提示模式）',
+    );
+    // 按钮复位可重试。
+    expect(find.text('Download poster image'), findsOneWidget);
+
+    await tester.ensureVisible(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+    await tester.tap(find.text('Download poster image'));
+    await tester.pumpAndSettle();
+    expect(calls, 2, reason: '失败后可重试');
+  });
+
   testWidgets('Copy link（主操作）→ 剪贴板收到带 lang 的 URL + 已复制提示',
       (tester) async {
     final clipboardCalls = <MethodCall>[];