feat: Tab栏和KPI卡片吸顶固定

滚动列表时：
- Tab栏（实时监控/统计报表/每日汇报）sticky固定在顶部
- KPI统计卡片sticky固定在Tab栏下方，略缩小间距
- 背景色匹配页面避免透出内容

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/modules/mileage/MileageModule.tsx

@@ -11,8 +11,8 @@ export default function MileageModule() {
   return (
     <div className="min-h-screen bg-[#F8F9FB] text-gray-800 font-sans p-3 md:p-6 relative overflow-x-hidden">
       <div className="max-w-6xl mx-auto flex flex-col gap-3 pb-16 landscape:pb-0 landscape:h-full landscape:flex-1 landscape:overflow-hidden">
-        {/* Sub-navigation */}
-        <div className="bg-white px-4 py-2 rounded-2xl border border-slate-100 shadow-sm flex items-center gap-6">
+        {/* Sub-navigation — sticky */}
+        <div className="bg-white px-4 py-2 rounded-2xl border border-slate-100 shadow-sm flex items-center gap-6 sticky top-0 z-30">
           <button
             onClick={() => setActiveSubTab('monitoring')}
             className={`flex items-center gap-2 py-1 transition-all relative ${activeSubTab === 'monitoring' ? 'text-blue-600' : 'text-slate-400'}`}