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/components/ui/Modal.tsx

@@ -0,0 +1,34 @@
+import { useEffect, type ReactNode } from 'react';
+
+interface ModalProps {
+  open: boolean;
+  onClose: () => void;
+  children: ReactNode;
+}
+
+export function Modal({ open, onClose, children }: ModalProps) {
+  useEffect(() => {
+    if (!open) return;
+    const handler = (e: KeyboardEvent) => { if (e.key === 'Escape') onClose(); };
+    window.addEventListener('keydown', handler);
+    return () => window.removeEventListener('keydown', handler);
+  }, [open, onClose]);
+
+  if (!open) return null;
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center"
+      style={{ backgroundColor: 'rgba(0,0,0,0.4)' }}
+      onClick={onClose}
+    >
+      <div
+        className="rounded-lg p-6"
+        style={{ backgroundColor: 'var(--app-bg)', color: 'var(--app-fg)' }}
+        onClick={(e) => e.stopPropagation()}
+      >
+        {children}
+      </div>
+    </div>
+  );
+}