feat: 前台页头加明暗切换，刷新图标换成闪电

- settings.site.actions 新增 theme（默认开），后台「操作工具」可关
- 前台页头：月亮/太阳按钮，点击写回 site.theme，前后台同步；
  未登录时不显示（设置接口需要会话）
- 刷新图标 refresh_3_line -> lightning_line，刷新期间仍旋转

stash-web/src/AppNext.tsx

@@ -6,6 +6,7 @@ import {
   SearchField,
   Space,
   Toaster,
+  toast,
   type MenuItem,
 } from '@stash/ui'
 import { useEffect, useMemo, useRef, useState } from 'react'
@@ -26,6 +27,7 @@ import { useProjects } from './useProjects'
 import { fetchSession, logout, type SessionState } from './auth'
 import {
   fetchSettings,
+  saveSettings,
   type SettingsDocument,
   type SiteSettings,
 } from './settings'
@@ -287,7 +289,7 @@ export default function AppNext() {
       favicon: '',
       theme: 'auto',
       brand: { name: 'STASH', mark: 'S', logo: '' },
-      actions: { refresh: true, settings: true, session: true },
+      actions: { theme: true, refresh: true, settings: true, session: true },
     },
   }
 
@@ -298,6 +300,27 @@ export default function AppNext() {
     document.documentElement.dataset.scheme = site.site.theme
   }, [site.site.theme])
 
+  /** 当前实际是深色吗（auto 时按系统偏好算） */
+  const isDark =
+    site.site.theme === 'dark' ||
+    (site.site.theme === 'auto' &&
+      window.matchMedia('(prefers-color-scheme: dark)').matches)
+
+  /** 页头的明暗切换：写回站点设置，前后台一起变；只有登录后才会显示 */
+  const toggleTheme = async () => {
+    if (settings === null) return
+    const next = isDark ? 'light' : 'dark'
+    const result = await saveSettings(settings.version, {
+      ...settings.settings,
+      site: { ...settings.settings.site, theme: next },
+    })
+    if (result.ok) {
+      setSettings(result.document)
+      return
+    }
+    toast.error(result.error)
+  }
+
   // 站点级设置：浏览器标签标题与 favicon（空值回落到内置）
   useEffect(() => {
     const title = site.site.title.trim()
@@ -648,6 +671,8 @@ export default function AppNext() {
             authenticated={session?.authenticated === true}
             username={session?.username ?? undefined}
             refreshing={refreshing}
+            dark={isDark}
+            onToggleTheme={() => void toggleTheme()}
             onRefresh={() => {
               refresh()
               load()