feat: 图片预览与发送全量实现 (#31~#35)

- #31 ImageMessageBubble: typ=2 气泡，max 220pt / min 80pt 尺寸规则，进度环叠层
- #32 ImageViewerPage: photo_view 全屏查看，PhotoViewGallery 多图滑动，保存+分享工具栏
- #33 ImagePickerSheet + SendImageUseCase: 相册/相机选图(最多9张)，裁剪，dart:ui 解析宽高，FormData CDN 上传，typ=2 发送
- #34 image_cropper 接入：_PreviewTile 点击裁剪，iOS TOCropViewController 对齐
- #35 _MessageBubble BubbleKind 路由：typ switch → 各媒体气泡，输入栏新增附件按钮

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

apps/im_app/lib/features/chat/usecases/send_image_usecase.dart

@@ -0,0 +1,106 @@
+import 'dart:io';
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+
+import 'package:flutter/foundation.dart';
+import 'package:networks_sdk/networks_sdk.dart';
+
+import 'package:im_app/data/remote/upload_file_request.dart';
+import 'package:im_app/features/chat/usecases/send_message_use_case.dart';
+
+/// 图片上传并发送消息用例（#33）
+///
+/// 对应 iOS `ImageProcessor.swift` + `ChatView.swift sendImageMessage()`
+///
+/// ## 执行流程
+/// 1. 读取图片字节 → [Uint8List]
+/// 2. `dart:ui.ImageDescriptor.encoded()` → 解析压缩后宽高（零外部依赖）
+/// 3. 写入临时文件 → [UploadFileRequest] FormData → [UploadResult.url]
+/// 4. `jsonEncode({"url":url,"width":w,"height":h})` → [SendMessageUseCase] typ=2
+/// 5. 删除临时文件
+///
+/// 发送中进度通过 [onProgress] 回调传出（0.0~1.0），
+/// 用于 [ImageMessageBubble] 渲染进度环。
+class SendImageUseCase {
+  final NetworksSdkApi _apiClient;
+  final SendMessageUseCase _sendMessage;
+
+  SendImageUseCase({
+    required NetworksSdkApi apiClient,
+    required SendMessageUseCase sendMessage,
+  })  : _apiClient = apiClient,
+        _sendMessage = sendMessage;
+
+  Future<void> execute({
+    required String filePath,
+    required int chatId,
+    int chatType = 1,
+    void Function(double)? onProgress,
+  }) async {
+    // 1. 读取字节
+    final bytes = await File(filePath).readAsBytes();
+
+    // 2. 解析宽高
+    final (width, height) = await _resolveSize(bytes);
+
+    // 3. 写临时文件（image_picker 有时返回没有扩展名的路径，确保临时文件有扩展名）
+    final ext = _extOf(filePath);
+    final tempFile = File(
+      '${Directory.systemTemp.path}/upload_${DateTime.now().millisecondsSinceEpoch}$ext',
+    );
+    await tempFile.writeAsBytes(bytes);
+
+    onProgress?.call(0.1);
+
+    // 4. 上传
+    String uploadedUrl = '';
+    try {
+      final result = await _apiClient.executeRequest(
+        UploadFileRequest(filePath: tempFile.path),
+      );
+      uploadedUrl = result?.url ?? '';
+    } catch (e) {
+      debugPrint('[SendImageUseCase] upload error: $e');
+      rethrow;
+    } finally {
+      // 5. 删临时文件（无论成败）
+      try {
+        await tempFile.delete();
+      } catch (_) {}
+    }
+
+    if (uploadedUrl.isEmpty) {
+      throw Exception('[SendImageUseCase] upload returned empty url');
+    }
+
+    onProgress?.call(0.9);
+
+    // 6. 发送 typ=2 消息
+    final content = '{"url":"$uploadedUrl","width":$width,"height":$height}';
+    await _sendMessage.execute(chatId: chatId, content: content, typ: 2);
+
+    onProgress?.call(1.0);
+  }
+
+  /// 使用 [dart:ui.ImageDescriptor] 解析字节宽高（零额外依赖）
+  Future<(int, int)> _resolveSize(Uint8List bytes) async {
+    try {
+      final buffer = await ui.ImmutableBuffer.fromUint8List(bytes);
+      final descriptor = await ui.ImageDescriptor.encoded(buffer);
+      final w = descriptor.width;
+      final h = descriptor.height;
+      descriptor.dispose();
+      buffer.dispose();
+      return (w, h);
+    } catch (e) {
+      debugPrint('[SendImageUseCase] resolveSize error: $e');
+      return (0, 0);
+    }
+  }
+
+  String _extOf(String path) {
+    final dot = path.lastIndexOf('.');
+    if (dot == -1 || dot == path.length - 1) return '.jpg';
+    return path.substring(dot);
+  }
+}