feat: 前台明暗切换改为本机偏好，不再要求登录

- 新增 src/theme.ts：localStorage 的 stash-theme 作为本机偏好，优先于站点默认
- 前台页头的月亮/太阳只写本机偏好，未登录也能用，刷新后保持
- 后台「页头 → 配色」与后台页头按钮改的是站点默认值；后台按钮同时清掉本机偏好，
  保证点完看到的就是新默认
- 两个壳都按「本机偏好 ?? 站点默认」决定 data-scheme

stash-web/src/AppNext.tsx

@@ -6,7 +6,6 @@ import {
   SearchField,
   Space,
   Toaster,
-  toast,
   type MenuItem,
 } from '@stash/ui'
 import { useEffect, useMemo, useRef, useState } from 'react'
@@ -27,10 +26,15 @@ import { useProjects } from './useProjects'
 import { fetchSession, logout, type SessionState } from './auth'
 import {
   fetchSettings,
-  saveSettings,
   type SettingsDocument,
   type SiteSettings,
 } from './settings'
+import {
+  isDarkScheme,
+  readThemeOverride,
+  writeThemeOverride,
+  type ThemeChoice,
+} from './theme'
 import {
   appDetailModel,
   posterDetailModel,
@@ -295,30 +299,25 @@ export default function AppNext() {
 
   const keywordLower = keyword.trim().toLowerCase()
 
-  // 配色：写在 <html data-scheme> 上，深色由组件库的令牌块接管
+  // 配色：本机偏好优先于站点默认，写在 <html data-scheme> 上，
+  // 深色由组件库的令牌块接管，页面与组件都不用改
+  const [themeOverride, setThemeOverride] = useState<ThemeChoice | null>(() =>
+    readThemeOverride(),
+  )
+  const effectiveTheme = themeOverride ?? site.site.theme
+
   useEffect(() => {
-    document.documentElement.dataset.scheme = site.site.theme
-  }, [site.site.theme])
+    document.documentElement.dataset.scheme = effectiveTheme
+  }, [effectiveTheme])
 
   /** 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)
+  const isDark = isDarkScheme(effectiveTheme)
 
-  /** 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)
+  /** 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 = () => {
+    const next: ThemeChoice = isDark ? 'light' : 'dark'
+    setThemeOverride(next)
+    writeThemeOverride(next)
   }
 
   // 站点级设置：浏览器标签标题与 favicon（空值回落到内置）

stash-web/src/components/admin/AdminNext.tsx

@@ -26,6 +26,7 @@ import { useEffect, useMemo, useState } from 'react'
 import { changePassword, fetchSession, logout, type SessionState } from '../../auth'
 import { LoginDialog } from '../auth/LoginDialog'
 import { GroupMark as GroupGlyph } from '../common/GroupMark'
+import { isDarkScheme, readThemeOverride, writeThemeOverride } from '../../theme'
 import {
   fetchSettings,
   saveSettings,
@@ -135,7 +136,8 @@ export function AdminNext() {
     if (meta === undefined) return
     const title = meta.title.trim()
     if (title !== '') window.document.title = title
-    window.document.documentElement.dataset.scheme = meta.theme
+    // 本机偏好优先于站点默认，跟前台一致
+    window.document.documentElement.dataset.scheme = readThemeOverride() ?? meta.theme
     const href = meta.favicon.trim() === '' ? '/favicon.svg' : meta.favicon.trim()
     let link = window.document.head.querySelector<HTMLLinkElement>('link[rel="icon"]')
     if (link === null) {
@@ -178,15 +180,15 @@ export function AdminNext() {
     toast.success('侧边栏已保存')
   }
 
-  /** 页头的明暗切换：写回 site.theme，前后台一起变 */
+  /** 页头的明暗切换：改的是站点默认值，并清掉本机偏好，保证点完看到的就是新默认 */
   const themeSetting = draft?.site.theme ?? document?.settings.site.theme ?? 'auto'
-  const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches
-  const isDark = themeSetting === 'dark' || (themeSetting === 'auto' && prefersDark)
+  const isDark = isDarkScheme(readThemeOverride() ?? themeSetting)
 
   const toggleTheme = async () => {
     if (document === null || draft === null) return
     const next = isDark ? 'light' : 'dark'
     setSaving(true)
+    writeThemeOverride(null)
     const result = await saveSettings(document.version, {
       ...draft,
       site: { ...draft.site, theme: next },

stash-web/src/theme.ts

@@ -0,0 +1,35 @@
+/**
+ * 明暗主题的「本机偏好」。
+ *
+ * 站点设置里的 `site.theme` 是**所有人**的默认值；这里的 localStorage 是**这台浏览器**
+ * 的选择，读到时优先于默认值。前台页头的切换按钮只改本机偏好，不需要登录；
+ * 后台「页头 → 配色」改的是站点默认值。
+ */
+export type ThemeChoice = 'light' | 'dark'
+
+const STORAGE_KEY = 'stash-theme'
+
+export function readThemeOverride(): ThemeChoice | null {
+  try {
+    const value = window.localStorage.getItem(STORAGE_KEY)
+    return value === 'light' || value === 'dark' ? value : null
+  } catch {
+    return null
+  }
+}
+
+export function writeThemeOverride(value: ThemeChoice | null): void {
+  try {
+    if (value === null) window.localStorage.removeItem(STORAGE_KEY)
+    else window.localStorage.setItem(STORAGE_KEY, value)
+  } catch {
+    // 隐私模式下 localStorage 可能不可写，忽略即可
+  }
+}
+
+/** `light` / `dark` / `auto` 里，当前实际是不是深色 */
+export function isDarkScheme(scheme: string): boolean {
+  if (scheme === 'dark') return true
+  if (scheme === 'light') return false
+  return window.matchMedia('(prefers-color-scheme: dark)').matches
+}