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/message-pagination.test.ts

@@ -20,4 +20,14 @@ describe('message pagination', () => {
     )
     expect(merged.map((message) => message.id)).toEqual(['oldest', 'overlap', 'latest'])
   })
+
+  it('keeps cross-year pages continuous through the earliest fixture record', () => {
+    const page2025 = [makeMessage('2025', 1_735_689_600), makeMessage('2026', 1_767_225_600)]
+    const page2017 = [makeMessage('2017', 1_483_228_800), makeMessage('2025', 1_735_689_600)]
+
+    const merged = mergeMessagePages(page2017, page2025)
+
+    expect(merged.map((message) => message.id)).toEqual(['2017', '2025', '2026'])
+    expect(new Set(merged.map((message) => message.id)).size).toBe(merged.length)
+  })
 })