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

tests/unit/connection-diagnostics.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest'
+import { buildSafeDiagnosticSummary } from '../../src/shared/connection-diagnostics'
+
+describe('connection diagnostics', () => {
+  it('contains useful versions and readiness without secrets or full account paths', () => {
+    const summary = buildSafeDiagnosticSummary({
+      platform: 'win32',
+      osVersion: 'Windows 11 fixture',
+      appVersion: 'v2.1.6',
+      wechatVersion: '4.1.9.57',
+      dataStructureVersion: '微信 4.x（WCDB）',
+      dataDirectoryDetected: true,
+      autoDetectSupported: true,
+      wechatRunning: true,
+      accountIdentified: true,
+      dbConnected: false,
+      encryptionAvailable: true
+    })
+
+    expect(summary).toContain('WechatExplorer: v2.1.6')
+    expect(summary).toContain('微信客户端: 4.1.9.57')
+    expect(summary).not.toContain('0123456789abcdef')
+    expect(summary).not.toContain('C:\\Users\\fixture\\xwechat_files\\wxid_secret')
+    expect(summary).not.toContain('wxid_')
+  })
+})