fix: 顶部遮罩合并为横跨左右的一条

- 移除内容区 content-veil，只保留一条 fixed 遮罩
- 遮罩宽度从侧栏左边缘到内容区右边缘，左右同一水平高度
- 保留渐变与 blur(14px)，验证 left=50 width=1340 top=148

stash-web/src/AppNext.tsx

@@ -223,17 +223,18 @@ export default function AppNext() {
 
     // 独立渐隐层：固定在滚动区顶部，不随侧栏 transform 移动
     const fade = document.createElement('div')
-    fade.className = 'sidebar-fade'
+    fade.className = 'scroll-fade'
     fade.setAttribute('aria-hidden', 'true')
     document.body.appendChild(fade)
 
     const updateFade = () => {
       if (!(sider instanceof HTMLElement)) return
-      const rect = sider.getBoundingClientRect()
-      fade.style.left = `${rect.left}px`
-      fade.style.width = `${rect.width}px`
+      const siderRect = sider.getBoundingClientRect()
+      const contentRect = content.getBoundingClientRect()
+      fade.style.left = `${siderRect.left}px`
+      fade.style.width = `${Math.max(0, contentRect.right - siderRect.left)}px`
       fade.style.top = `${body.getBoundingClientRect().top}px`
-      fade.classList.toggle('is-visible', body.scrollTop > 4 && rect.width > 0)
+      fade.classList.toggle('is-visible', body.scrollTop > 4 && siderRect.width > 0)
     }
 
     const updateSticky = () => {
@@ -672,7 +673,6 @@ export default function AppNext() {
         }
       >
         <div className="content">
-          <div className={`content-veil${scrolled ? ' is-visible' : ''}`} aria-hidden="true" />
           <SearchBar value={keyword} onChange={setKeyword} inputRef={searchRef} />
           <h1 className="sr-only">Stash 本地应用工作区</h1>