refactor: 抽取 Modal 组件、支持 JSON/CSV 导出、清理冗余代码

- 新增 Modal 组件，消除 3 个 Dialog 中重复的遮罩层/Escape/stopPropagation 代码
- PathEditDialog/HelpDialog/ImportDialog 改用 Modal 包裹
- handleExport 支持 JSON/CSV 两种格式（CSV 导出代码之前存在但从未接线）
- App.tsx 移除冗余的 initDarkMode 后重复设 store 的逻辑
- ErrorBoundary 添加 componentDidCatch 日志和 console.error

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/App.tsx

@@ -1,6 +1,6 @@
 import { useEffect } from 'react';
 import { useAppStore } from '@/store/app-store';
-import { initDarkMode, useThemeStore } from '@/store/theme-store';
+import { initDarkMode } from '@/store/theme-store';
 import { AppShell } from '@/components/layout/AppShell';
 import { ErrorBoundary } from '@/components/layout/ErrorBoundary';
 
@@ -9,10 +9,6 @@ export default function App() {
 
   useEffect(() => {
     initDarkMode();
-    const saved = localStorage.getItem('darkMode');
-    if (saved === '1') {
-      useThemeStore.setState({ isDark: true });
-    }
     initialize();
   }, [initialize]);