feat: 完善多账号连接诊断与聊天媒体导出

- 新增微信账号发现、环境诊断和分步数据库连接引导
- 支持按账号安全保存数据库密钥及快速切换账号
- 完善 WCDB 历史消息分片读取和分页状态提示
- 支持导出图片、视频和语音，提供原图优先及缩略图回退
- 更新安装指引、兼容版本说明和相关自动化测试

src/shared/database-key.ts

@@ -24,8 +24,36 @@ export interface DatabaseKeyStorageResult {
   encryptionAvailable: boolean
 }
 
+export type AccountLoginStatus = 'current' | 'other' | 'unknown'
+
+export interface WechatAccountCandidate {
+  id: string
+  accountRoot: string
+  directoryName: string
+  wxid?: string
+  nickname?: string
+  avatar?: string
+  hasSavedDbKey: boolean
+  loginStatus: AccountLoginStatus
+  selectedByInput: boolean
+}
+
+export interface AccountDiscoveryResult {
+  success: boolean
+  inputKind?: 'root' | 'account'
+  accounts: WechatAccountCandidate[]
+  preselectedAccountId?: string
+  error?: string
+}
+
 export interface DatabaseKeyEnvironment {
   platform: NodeJS.Platform
+  osVersion: string
+  appVersion: string
+  wechatVersion: string
+  dataStructureVersion: string
+  dataDirectoryDetected: boolean
+  diagnosticSummary: string
   autoDetectSupported: boolean
   wechatRunning: boolean
   accountIdentified: boolean