fix(roundtable): 修复弹窗被顶部导航栏遮挡的 z-index 问题

将全局 Header z-[2101] 降为 z-[45]、圆桌页内部 header z-[2000] 降为 z-[40]，
使两者低于 Shadcn Dialog 遮罩层 z-50，弹窗打开时可正常覆盖导航栏。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend-web/src/roundtable-planning/pages/RoundtablePlanningPage.tsx

@@ -1366,7 +1366,7 @@ export default function RoundtablePlanningPage() {
       )}
 
       {/* HEADER */}
-      <header className="relative z-[2000] flex shrink-0 items-center justify-between gap-4 border-b border-slate-100 bg-white px-6 py-3 shadow-[0_1px_3px_rgba(0,0,0,0.02)] dark:border-border dark:bg-card dark:shadow-none">
+      <header className="relative z-[40] flex shrink-0 items-center justify-between gap-4 border-b border-slate-100 bg-white px-6 py-3 shadow-[0_1px_3px_rgba(0,0,0,0.02)] dark:border-border dark:bg-card dark:shadow-none">
         <StepNavigation
           currentStep={currentStep}
           intentReady={step1.intentReady}

frontend-web/src/strategy-components/Header.tsx

@@ -294,7 +294,7 @@ const Header = ({ collapsed }: HeaderProps) => {
         open={notificationOpen}
         onOpenChange={setNotificationOpen}
       />
-      <header className="strategy-app-header theme-transition relative z-[2101] flex h-16 items-center justify-between px-4">
+      <header className="strategy-app-header theme-transition relative z-[45] flex h-16 items-center justify-between px-4">
         {showTopNav ? (
           <>
             {/* 左分区（flex-1）：菜单靠右贴近中间基座 */}