fix(electron-demo): 修复 Vault 保存路径边界校验

[hamroun1]

概述

修复了 Vault 保存路径校验逻辑，避免具有相同前缀的兄弟目录被错误识别为 Vault 内部路径。

修改内容

• 新增 isPathInside 路径校验工具函数。
• 使用 isPathInside 替换原来的 startsWith 路径判断。
• 新增单元测试，覆盖以下场景：
  • 相同路径
  • 子目录路径
  • 尾部斜杠
  • Windows 路径分隔符
  • /vault 与 /vault2 前缀冲突
  • 根目录路径

验证

已通过以下验证：

• pnpm vitest run apps/electron-demo/test/path-utils.test.ts
• pnpm typecheck

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