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/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 },