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）