fix(theme): 消除首帧主题闪动并柔化明暗过渡

- index.html 首帧按本机偏好设置 dark/light，避免先浅后深
- 实际 scheme 只写 dark/light（之前 auto 会覆盖成浅色）
- 切换时给所有表面加 240ms 颜色/边框/阴影过渡，结束后移除
- 背景图深色下淡出而不是 display:none
- 页头操作图标与分组导航等设置加载后再渲染，避免先显示后消失

stash-web/src/AppNext.tsx

@@ -300,13 +300,21 @@ export default function AppNext() {
     readThemeOverride(),
   )
   const effectiveTheme = themeOverride ?? site.site.theme
-
-  useEffect(() => {
-    document.documentElement.dataset.scheme = effectiveTheme
-  }, [effectiveTheme])
-
   /** 当前实际是深色吗（auto 时按系统偏好算） */
   const isDark = isDarkScheme(effectiveTheme)
+  const scheme = isDark ? 'dark' : 'light'
+
+  useEffect(() => {
+    const root = document.documentElement
+    if (root.dataset.scheme === scheme) return
+    root.classList.add('theme-transition')
+    root.dataset.scheme = scheme
+    const timer = window.setTimeout(() => root.classList.remove('theme-transition'), 260)
+    return () => {
+      window.clearTimeout(timer)
+      root.classList.remove('theme-transition')
+    }
+  }, [scheme])
 
   /** 页头的明暗切换：只写本机偏好，不需要登录；站点默认值在后台改 */
   const toggleTheme = () => {
@@ -663,6 +671,7 @@ export default function AppNext() {
             headerSpeed={site.header.speed}
             headerGap={site.header.gap}
             site={site.site}
+            ready={settings !== null}
             authenticated={session?.authenticated === true}
             username={session?.username ?? undefined}
             refreshing={refreshing}
@@ -707,7 +716,7 @@ export default function AppNext() {
                   onOpenApi={() => navigate('/interface')}
                 />
               )}
-              {settings !== null && site.sidebar.show_nav !== false && (
+              {settings !== null && !loading && site.sidebar.show_nav !== false && (
                 <div className="nav-card">
                   <Menu
                     items={sidebarItems}