fix: 去掉顶部遮罩，页头与搜索框改为半透明

- 删除 scroll-fade 遮罩及创建逻辑
- 页头改为 blur 并降到 55% 透明度，搜索框降到 60%
- 背景图渐隐层从 80% 调到 65%，顶部区域保持背景原色
- 验证：fades=0，header alpha=0.55，search alpha=0.60

stash-web/src/AppNext.tsx

@@ -221,22 +221,6 @@ export default function AppNext() {
 
     const onScroll = () => setScrolled(body.scrollTop > 4)
 
-    // 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 = 'scroll-fade'
-    fade.setAttribute('aria-hidden', 'true')
-    document.body.appendChild(fade)
-
-    const updateFade = () => {
-      if (!(sider instanceof HTMLElement)) return
-      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 = `${body.getBoundingClientRect().top}px`
-      fade.classList.toggle('is-visible', body.scrollTop > 4 && siderRect.width > 0)
-    }
-
     const updateSticky = () => {
       const viewport = body.clientHeight
       const maxLeft = Math.max(0, sider instanceof HTMLElement ? sider.offsetHeight - viewport : 0)
@@ -244,7 +228,6 @@ export default function AppNext() {
       if (sider instanceof HTMLElement) {
         sider.style.transform = offset > 0 ? `translateY(${offset}px)` : ''
       }
-      updateFade()
     }
 
     body.addEventListener('scroll', onScroll, { passive: true })
@@ -261,7 +244,6 @@ export default function AppNext() {
       body.removeEventListener('scroll', updateSticky)
       window.removeEventListener('resize', updateSticky)
       observer.disconnect()
-      fade.remove()
       body.id = ''
       body.style.overflowY = ''
       body.style.overflowX = ''
@@ -618,7 +600,7 @@ export default function AppNext() {
         siderWidth={240}
         gap={20}
         headerHeight={60}
-        headerVariant="solid"
+        headerVariant="blur"
         siderBordered={false}
         contentPadding="none"
         contentOffset={88}