feat(client): 商品详情页图片支持双击全屏查看

抽出共享全屏查看器 fullscreen_image_viewer.dart（复用扫码页的
InteractiveViewer+PageView+黑底 modal），详情页缩略图双击打开，
扫码公开页改为复用同一组件去重。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/screens/public/public_product_screen.dart

@@ -6,6 +6,7 @@ import 'package:flutter/services.dart';
 import 'package:go_router/go_router.dart';
 import 'package:url_launcher/url_launcher.dart';
 import '../../core/config/app_config.dart';
+import '../../widgets/fullscreen_image_viewer.dart';
 
 // ── Palette & typography constants ────────────────────────────────────
 const _kPaper = Color(0xFFFAF8F5);
@@ -476,12 +477,7 @@ class _HeroGalleryState extends State<_HeroGallery> {
   }
 
   void _openFullscreen(int index) {
-    Navigator.of(context).push(PageRouteBuilder(
-      opaque: false,
-      barrierColor: Colors.black87,
-      pageBuilder: (_, __, ___) => _FullscreenViewer(
-        urls: widget.imageUrls, initialIndex: index),
-    ));
+    showFullscreenImages(context, widget.imageUrls, initialIndex: index);
   }
 }
 
@@ -534,66 +530,6 @@ class _GlassChip extends StatelessWidget {
   }
 }
 
-// ── Full-screen image viewer ──────────────────────────────────────────
-
-class _FullscreenViewer extends StatefulWidget {
-  final List<String> urls;
-  final int initialIndex;
-  const _FullscreenViewer({required this.urls, required this.initialIndex});
-
-  @override
-  State<_FullscreenViewer> createState() => _FullscreenViewerState();
-}
-
-class _FullscreenViewerState extends State<_FullscreenViewer> {
-  late PageController _ctrl;
-
-  @override
-  void initState() {
-    super.initState();
-    _ctrl = PageController(initialPage: widget.initialIndex);
-  }
-
-  @override
-  void dispose() { _ctrl.dispose(); super.dispose(); }
-
-  @override
-  Widget build(BuildContext context) {
-    return GestureDetector(
-      onTap: () => Navigator.of(context).pop(),
-      child: Scaffold(
-        backgroundColor: Colors.transparent,
-        body: Stack(
-          children: [
-            PageView.builder(
-              controller: _ctrl,
-              itemCount: widget.urls.length,
-              itemBuilder: (_, i) => Center(
-                child: InteractiveViewer(
-                  child: Image.network(widget.urls[i], fit: BoxFit.contain,
-                      errorBuilder: (_, __, ___) =>
-                          const Icon(Icons.broken_image, size: 64, color: Colors.white54)),
-                )),
-            ),
-            Positioned(
-              top: 40, right: 16,
-              child: GestureDetector(
-                onTap: () => Navigator.of(context).pop(),
-                child: Container(
-                  padding: const EdgeInsets.all(8),
-                  decoration: BoxDecoration(
-                    color: Colors.black54, borderRadius: BorderRadius.circular(20)),
-                  child: const Icon(Icons.close, color: Colors.white, size: 20),
-                ),
-              ),
-            ),
-          ],
-        ),
-      ),
-    );
-  }
-}
-
 // ── Verified Ribbon ───────────────────────────────────────────────────
 
 class _VerifiedRibbon extends StatelessWidget {