feat: 重构为前后端分离架构并完善设备端演示

- 新增 Web 用户端（登录体系 + 统一 REST API 调用）
- 后端增加用户认证、统一 ApiResponse、CORS 支持
- Android 设备端迁移至 MVVM + DataBinding + Retrofit 网络层
- 完善 README 架构说明与密码学原理文档
- 新增 .gitignore 与持久化数据表说明

android-app/local.properties

@@ -1,10 +1,8 @@
-## This file is automatically generated by Android Studio.
-# Do not modify this file -- YOUR CHANGES WILL BE ERASED!
-#
-# This file should *NOT* be checked into Version Control Systems,
+## This file must *NOT* be checked into Version Control Systems,
 # as it contains information specific to your local configuration.
 #
 # Location of the SDK. This is only used by Gradle.
 # For customization when using a Version Control System, please read the
 # header note.
-sdk.dir=F\:\\AndroidSDK
+#Thu Aug 20 17:37:36 CST 2026
+sdk.dir=/mnt/sdk/AndroidSdk