fix(mobile): add top tab switcher and remove dead settings entry

Sidebar was hidden on mobile with no fallback, so users were stuck on
the default view. Add a horizontal tab strip under the header for
mobile viewports and drop the "后台系统设置" button which had no handler
or matching view.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -1,7 +1,8 @@
 import { useState } from 'react';
 import { Dashboard } from './components/Dashboard';
-import { Sidebar } from './components/Sidebar';
+import { Sidebar, sidebarMenuItems } from './components/Sidebar';
 import { Header } from './components/Header';
+import { cn } from './lib/utils';
 
 export default function App() {
   const [currentView, setCurrentView] = useState('overall');
@@ -17,6 +18,28 @@ export default function App() {
       <Sidebar currentView={currentView} setCurrentView={setCurrentView} />
       <div className="flex flex-col flex-1 overflow-hidden">
         <Header title={viewTitles[currentView] || '数据分析'} />
+        <nav className="md:hidden bg-white border-b border-slate-200 px-2 shrink-0 overflow-x-auto">
+          <div className="flex gap-1 min-w-max">
+            {sidebarMenuItems.map((item) => {
+              const active = currentView === item.id;
+              return (
+                <button
+                  key={item.id}
+                  onClick={() => setCurrentView(item.id)}
+                  className={cn(
+                    "flex items-center gap-1.5 px-3 py-2.5 text-xs font-semibold whitespace-nowrap border-b-2 transition-colors",
+                    active
+                      ? "text-indigo-600 border-indigo-500"
+                      : "text-slate-500 border-transparent active:text-slate-700"
+                  )}
+                >
+                  <item.icon className={cn("w-4 h-4", active ? "text-indigo-500" : "text-slate-400")} />
+                  {item.name}
+                </button>
+              );
+            })}
+          </div>
+        </nav>
         <main className="flex-1 overflow-y-auto p-4 md:p-6 lg:p-8">
           <Dashboard currentView={currentView} />
         </main>