fix: 统一弹框组件 — ModalOverlay对齐EditInstanceDialog行为

- ModalOverlay移除遮罩层onClick(不响应外部点击关闭)
- 移除body.style.overflow=hidden(修复白条问题)
- max-h从90vh改为85vh(与EditInstanceDialog一致)
- 10个页面手写fixed弹框替换为ModalOverlay组件
- InstanceGroupPage/WorkflowPage/UsersPage/RedisDetailPage/TicketsPage已替换
- dialog.tsx添加onPointerDownOutside/onInteractOutside阻止外部点击
- index.css添加body[data-scroll-locked]覆盖Radix滚动锁定
- Layout.tsx main添加relative定位上下文

frontend/src/components/Layout.tsx

@@ -122,7 +122,7 @@ export default function Layout() {
       </aside>
 
       {/* 主内容区 */}
-      <main className="flex-1 flex flex-col min-w-0 overflow-hidden">
+      <main id="app-main" className="relative flex-1 flex flex-col min-w-0 overflow-hidden">
         {/* 顶部栏 */}
         <header className="h-14 bg-[var(--dg-bg-secondary)] border-b border-[var(--dg-border)] flex items-center justify-between px-5 flex-shrink-0">
           {/* Breadcrumb area */}