feat(image): 图片查看全量升级 — 缓存/Hero/下拉关闭/长按菜单（#57~#59）

#57 cached_network_image 接入
- pubspec 新增 cached_network_image: ^3.3.1
- CachedNetworkImageProvider 替换 PhotoView 中的 NetworkImage
- 磁盘+内存双缓存，同 URL 第二次加载无网络请求

#58 ImageViewerPage 完整重写
- Shimmer 加载占位（灰色渐变动画 + 进度百分比）
- 加载失败重试按钮（_ErrorWidget）
- 下拉关闭：>80pt 松手 pop，背景随拖动渐变透明
- 长按底部菜单：保存 / 分享 / 复制链接
- AppBar 右上角"⋮"快捷菜单
- 多图页面指示点（≤10张，活跃项宽度扩展为18pt）
- Hero 动画（单图，heroTag: 'img_$url'）
- 点击切换 AppBar/工具栏显示/隐藏（沉浸式）
- 全屏沉浸模式（SystemUiMode.immersiveSticky）

#59 气泡接入 CachedNetworkImage
- ImageMessageBubble: Image.network → CachedNetworkImage + Hero tag
- ImageGridBubble._GridCell: Image.network → CachedNetworkImage
- 灰色 placeholder + 200ms fadeIn

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/im_app/lib/features/chat/view/widgets/image_grid_bubble.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 import 'dart:math';
 
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
 import 'package:im_app/core/services/cdn_url_resolver.dart';
@@ -133,14 +134,13 @@ class _GridCell extends StatelessWidget {
           width: size,
           height: size,
           child: url.isNotEmpty
-              ? Image.network(
-                  url,
+              ? CachedNetworkImage(
+                  imageUrl: url,
                   fit: BoxFit.cover,
-                  loadingBuilder: (_, child, progress) {
-                    if (progress == null) return child;
-                    return Container(color: Colors.grey.shade200);
-                  },
-                  errorBuilder: (_, __, ___) => Container(
+                  fadeInDuration: const Duration(milliseconds: 200),
+                  placeholder: (_, __) =>
+                      Container(color: Colors.grey.shade200),
+                  errorWidget: (_, __, ___) => Container(
                     color: Colors.grey.shade300,
                     child: const Center(
                       child: Icon(

apps/im_app/lib/features/chat/view/widgets/image_message_bubble.dart

@@ -1,6 +1,7 @@
 import 'dart:convert';
 import 'dart:math';
 
+import 'package:cached_network_image/cached_network_image.dart';
 import 'package:flutter/material.dart';
 
 import 'package:im_app/core/services/cdn_url_resolver.dart';
@@ -51,9 +52,15 @@ class ImageMessageBubble extends StatelessWidget {
     final displaySize = _computeDisplaySize(rawW, rawH);
     final resolvedUrl = url.isNotEmpty ? CdnUrlResolver.resolve(url) : '';
 
+    final heroTag = resolvedUrl.isNotEmpty ? 'img_$resolvedUrl' : null;
+
     return GestureDetector(
       onTap: resolvedUrl.isNotEmpty
-          ? () => ImageViewerPage.open(context, urls: [resolvedUrl])
+          ? () => ImageViewerPage.open(
+                context,
+                urls: [resolvedUrl],
+                heroTag: heroTag,
+              )
           : null,
       child: ClipRRect(
         borderRadius: BorderRadius.circular(12),
@@ -63,34 +70,26 @@ class ImageMessageBubble extends StatelessWidget {
           child: Stack(
             fit: StackFit.expand,
             children: [
-              // ── 图片内容 ─────────────────────────────────────────────────────
+              // ── 图片内容（CachedNetworkImage + Hero）───────────────────────
               if (resolvedUrl.isNotEmpty)
-                Image.network(
-                  resolvedUrl,
-                  fit: BoxFit.cover,
-                  loadingBuilder: (_, child, loadingProgress) {
-                    if (loadingProgress == null) return child;
-                    return Container(
+                Hero(
+                  tag: heroTag!,
+                  child: CachedNetworkImage(
+                    imageUrl: resolvedUrl,
+                    fit: BoxFit.cover,
+                    fadeInDuration: const Duration(milliseconds: 200),
+                    placeholder: (_, __) => Container(
                       color: Colors.grey.shade200,
-                      child: Center(
-                        child: CircularProgressIndicator(
-                          value: loadingProgress.expectedTotalBytes != null
-                              ? loadingProgress.cumulativeBytesLoaded /
-                                  loadingProgress.expectedTotalBytes!
-                              : null,
-                          strokeWidth: 2,
+                    ),
+                    errorWidget: (_, __, ___) => Container(
+                      color: Colors.grey.shade300,
+                      child: const Center(
+                        child: Icon(
+                          Icons.broken_image_outlined,
+                          color: Colors.grey,
+                          size: 32,
                         ),
                       ),
-                    );
-                  },
-                  errorBuilder: (_, __, ___) => Container(
-                    color: Colors.grey.shade300,
-                    child: const Center(
-                      child: Icon(
-                        Icons.broken_image_outlined,
-                        color: Colors.grey,
-                        size: 32,
-                      ),
                     ),
                   ),
                 )