feat(e2e): 端对端加密完全对齐老项目 — cipher_guard_sdk 修正 + EncryptionManager 集成

修正 cipher_guard_sdk 4 个关键密码学差异使其与老 Flutter 项目 (im-client-im-dev) 和 iOS
EncryptionManager 完全互操作：

1. AES: 显式 SIC/CTR 模式 + 16 zero-byte IV（原 SDK 用随机 IV + KDF 派生密钥）
2. RSA: bare RSAEngine 无 PKCS1 padding（原 SDK 用 PKCS1Encoding）
3. Session key: 32-char alphanumeric ASCII（原 SDK 用 base64 random bytes）
4. Wire format: base64(ciphertext) 无 IV 前缀

新增 EncryptionManager：
- Per-chat round-based key chain（最多 10 rounds/chat，FIFO 淘汰）
- 登录后自动 setup：RSA 密钥对生成/存储 + 公钥上传 + chat 密钥拉取解密
- API 集成：cipher/v2/key/my, key/set, chat/my
- 消息加密返回 JSON envelope {"round":N,"data":"<base64>"}
- 消息解密兼容 JSON envelope + legacy raw base64

集成到消息流：
- SendMessageUseCase: 发送前加密 content → wireContent
- WsMessageService: 收到消息后解密 content + lastMsg
- 无密钥时 fallback 到明文（对齐 iOS 行为）

注意：/app/api/cipher/v2/key/set 仍为预发布接口，仅测试阶段使用

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -1,6 +1,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:networks_sdk/networks_sdk.dart';
 
+import 'package:im_app/core/services/encryption_manager.dart';
 import 'package:im_app/data/remote/send_message_request.dart';
 import 'package:im_app/domain/entities/message.dart';
 import 'package:im_app/domain/repositories/chat_repository.dart';
@@ -25,6 +26,7 @@ class SendMessageUseCase {
   final NetworksSdkApi _apiClient;
   final MessageRepository _messageRepo;
   final ChatRepository _chatRepo;
+  final EncryptionManager? _encryptionManager;
   final int currentUid;
 
   SendMessageUseCase({
@@ -32,9 +34,11 @@ class SendMessageUseCase {
     required MessageRepository messageRepo,
     required ChatRepository chatRepo,
     required this.currentUid,
+    EncryptionManager? encryptionManager,
   })  : _apiClient = apiClient,
         _messageRepo = messageRepo,
-        _chatRepo = chatRepo;
+        _chatRepo = chatRepo,
+        _encryptionManager = encryptionManager;
 
   Future<void> execute({
     required int chatId,
@@ -56,13 +60,25 @@ class SendMessageUseCase {
       ),
     );
 
-    // 2. HTTP 发送
+    // 2. E2E 加密（对齐 iOS MessageHistoryService.sendMessage）
+    //    wireContent = EncryptionManager.encryptContent(content, chatId) ?? content
+    String wireContent = content;
+    if (_encryptionManager != null && _encryptionManager.isSetup) {
+      final encrypted =
+          await _encryptionManager.encryptContent(content, chatId: chatId);
+      if (encrypted != null) {
+        wireContent = encrypted;
+      }
+      // null = no key for chat → send plaintext (对齐老项目 fallback)
+    }
+
+    // 3. HTTP 发送
     SendMessageResponse? resp;
     try {
       resp = await _apiClient.executeRequest(
         SendMessageRequest(
           chatId: chatId,
-          content: content,
+          content: wireContent,
           typ: typ,
           sendTime: sendTime,
         ),