refactor: PlatformSplitLayout 应用 Stitch 设计规范

- 移除 section 的 border/shadow/rounded-[2rem]（No-Line Rule）
- 侧边栏和右面板移除 border，改用表面层级色差分离
- 背景改用 surface-container-low 色调
- 移除 metrics 芯片的边框
- IntakeMainLayout 移动端隐藏右面板

furniture-3d-system/src/app/intake/IntakeMainLayout.tsx

@@ -28,7 +28,7 @@ export function IntakeMainLayout({
       metrics={metrics}
       fitParentHeight
       className="!min-h-0 [&>main]:!pl-4 [&>main]:!pr-0 [&>main>section]:!mx-0 [&>main>section]:!h-full [&>main>section]:!max-w-none"
-      rightPanelClassName="!w-[224px] !min-w-[224px] !max-w-[224px] !overflow-hidden p-3"
+      rightPanelClassName="!w-[224px] !min-w-[224px] !max-w-[224px] !overflow-hidden p-3 max-lg:!hidden"
       leftContent={leftContent}
       rightContent={rightContent}
     />

furniture-3d-system/src/components/platform/PlatformSplitLayout.tsx

@@ -160,23 +160,22 @@ export function PlatformSplitLayout({
 
       <main
         className={cn(
-          'flex min-h-0 flex-1 overflow-hidden px-8',
-          'max-lg:!px-3 max-lg:!pb-0',
-          fitParentHeight ? 'pb-0' : 'pb-8',
+          'flex min-h-0 flex-1 overflow-hidden',
+          'max-lg:!px-0 max-lg:!pb-0',
+          fitParentHeight ? 'pb-0' : '',
         )}
       >
         <section
-          className="mx-auto flex min-h-0 w-full max-w-[1720px] flex-1 overflow-hidden max-lg:!mx-0 max-lg:!max-w-none max-lg:!rounded-none max-lg:!border-0 max-lg:!shadow-none rounded-[2rem] border border-white/60 shadow-apple-xl"
+          className="mx-auto flex min-h-0 w-full max-w-[1720px] flex-1 overflow-hidden max-lg:!mx-0 max-lg:!max-w-none max-lg:!rounded-none"
           style={{
-            backgroundColor: 'var(--color-surface-container)',
+            backgroundColor: 'var(--color-surface-container-low)',
           }}
         >
           {mainContent ? (
             <>
               <aside
-                className="w-[280px] max-w-[22vw] shrink-0 overflow-y-auto border-r p-3 max-lg:!w-full max-lg:!max-w-none"
+                className="w-[280px] max-w-[22vw] shrink-0 overflow-y-auto p-3 max-lg:!w-full max-lg:!max-w-none"
                 style={{
-                  borderColor: 'var(--color-separator)',
                   backgroundColor: 'var(--color-surface-container-lowest)',
                 }}
               >
@@ -190,11 +189,10 @@ export function PlatformSplitLayout({
           {rightContent ? (
             <aside
               className={cn(
-                'w-[var(--layout-right-panel-width)] max-w-[var(--layout-right-panel-max-width)] shrink-0 overflow-y-auto border-l p-0 max-lg:hidden',
+                'w-[var(--layout-right-panel-width)] max-w-[var(--layout-right-panel-max-width)] shrink-0 overflow-y-auto p-0 max-lg:hidden',
                 rightPanelClassName,
               )}
               style={{
-                borderColor: 'var(--color-separator)',
                 backgroundColor: 'var(--color-surface-container-lowest)',
               }}
             >