refactor: 迁移至 electron-vite 官方推荐目录结构

- electron/* -> src/main/* (主进程)
- electron/preload.ts -> src/preload/index.ts (预加载)
- src/electron-api.d.ts -> src/preload/index.d.ts (类型声明)
- src/* -> src/renderer/src/* (渲染进程)
- src/index.html -> src/renderer/index.html
- electron.vite.config.ts 简化为自动入口检测 (零配置)
- tsconfig.app.json -> tsconfig.web.json, 更新 includes/paths
- 构建产物 dist-electron/ -> out/ (修复 main/preload 输出覆盖 bug)
- 更新 package.json main 字段 + electron-builder files
- 更新 .gitignore + eslint ignores
- 更新 AGENTS.md 架构文档

.gitignore

@@ -1,5 +1,5 @@
 node_modules/
-dist-electron/
+out/
 release/
 dist/
 *.log
@@ -15,7 +15,7 @@ yarn-error.log*
 *.njsproj
 *.sln
 
-# Legacy codebase — archived Vue 2 version, not part of v2 build
+# Legacy codebase - archived Vue 2 version, not part of v2 build
 legacy/
 
 # Brainstorming artifacts