style: 修复侧栏分类圆角，顶部增加软虚化

- nav-card 补 12px 圆角与 overflow hidden
- 搜索框 50% 透明 + blur(14px)
- 新增 top-blur：blur(8px) + 上下 mask 渐隐，覆盖搜索与内容顶部，不出现硬线
- 验证：nav 圆角 12、搜索可点、虚化生效

stash-web/src/App.css

@@ -151,8 +151,33 @@
 
 /* 搜索框半透明，顶部区域透出背景 */
 .search-row > *:first-child {
-  background-color: color-mix(in srgb, var(--su-bg-card) 60%, transparent);
-  backdrop-filter: blur(10px);
+  background-color: color-mix(in srgb, var(--su-bg-card) 50%, transparent);
+  backdrop-filter: blur(14px);
+}
+
+/* 搜索区域软虚化：上下都用 mask 渐隐，不出现横向分割线 */
+.top-blur {
+  position: fixed;
+  top: 60px;
+  right: 0;
+  left: 0;
+  z-index: 4;
+  height: 120px;
+  pointer-events: none;
+  background: linear-gradient(
+    to bottom,
+    color-mix(in srgb, var(--su-bg-page) 22%, transparent) 0%,
+    transparent 80%
+  );
+  backdrop-filter: blur(8px);
+  mask-image: linear-gradient(to bottom, transparent 0%, #000 28%, #000 62%, transparent 100%);
+  -webkit-mask-image: linear-gradient(
+    to bottom,
+    transparent 0%,
+    #000 28%,
+    #000 62%,
+    transparent 100%
+  );
 }
 
 /* ---------- 两栏骨架 ---------- */
@@ -1339,6 +1364,11 @@ a.entry-href:hover {
   backdrop-filter: blur(14px) saturate(1.15);
 }
 
+.sidebar-home .nav-card {
+  overflow: hidden;
+  border-radius: var(--su-radius-md);
+}
+
 .sidebar-divider {
   margin: var(--su-space-2) 0;
   border-top: 1px dashed var(--su-border-strong);

stash-web/src/AppNext.tsx

@@ -223,6 +223,13 @@ export default function AppNext() {
 
     const onScroll = () => setScrolled(body.scrollTop > 4)
 
+    // 顶部软虚化层：覆盖搜索区域与内容顶部，边缘用 mask 渐隐，避免硬分割线
+    const blur = document.createElement('div')
+    blur.className = 'top-blur'
+    blur.setAttribute('aria-hidden', 'true')
+    const appRoot = document.querySelector('.app')
+    appRoot?.appendChild(blur)
+
     const updateSticky = () => {
       const viewport = body.clientHeight
       const maxLeft = Math.max(0, sider instanceof HTMLElement ? sider.offsetHeight - viewport : 0)
@@ -246,6 +253,7 @@ export default function AppNext() {
       body.removeEventListener('scroll', updateSticky)
       window.removeEventListener('resize', updateSticky)
       observer.disconnect()
+      blur.remove()
       body.id = ''
       body.style.overflowY = ''
       body.style.overflowX = ''