feat(admin): 增加背景管理

- settings 新增 background.light / background.dark，留空使用内置图
- 后台首页管理新增「背景管理」，两栏 URL 输入与实时预览，可一键恢复内置图
- 前台按设置覆盖明暗背景图，仍保留 65vh 与交叉淡入
- 保存/撤销沿用现有设置乐观锁流程

stash-web/src/App.css

@@ -1379,3 +1379,46 @@ html.theme-transition .site-background {
     transition: none !important;
   }
 }
+
+/* 后台：背景管理预览 */
+.background-grid {
+  display: grid;
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+  gap: var(--su-space-5);
+}
+
+.background-section {
+  display: flex;
+  flex-direction: column;
+  gap: var(--su-space-3);
+  min-width: 0;
+}
+
+.background-preview {
+  position: relative;
+  height: 180px;
+  border: 1px solid var(--su-border);
+  border-radius: var(--su-radius-md);
+  background-color: var(--su-bg-subtle);
+  background-position: center top;
+  background-repeat: no-repeat;
+  background-size: cover;
+  overflow: hidden;
+}
+
+.background-preview span {
+  position: absolute;
+  bottom: var(--su-space-3);
+  left: var(--su-space-3);
+  padding: 2px var(--su-space-2);
+  border-radius: var(--su-radius-pill);
+  background-color: rgb(0 0 0 / 55%);
+  color: #fff;
+  font-size: var(--su-font-size-xs);
+}
+
+@media (width <= 900px) {
+  .background-grid {
+    grid-template-columns: minmax(0, 1fr);
+  }
+}

stash-web/src/AppNext.tsx

@@ -276,6 +276,7 @@ export default function AppNext() {
       show_service: true,
       show_nav: true,
     },
+    background: { light: '', dark: '' },
     profile: {
       name: '本地管理员',
       role: '管理员',
@@ -644,8 +645,24 @@ export default function AppNext() {
         }}
       />
 
-      <div className="site-background site-background-light" aria-hidden="true" />
-      <div className="site-background site-background-dark" aria-hidden="true" />
+      <div
+        className="site-background site-background-light"
+        style={
+          site.background.light.trim() === ''
+            ? undefined
+            : { backgroundImage: `url(${JSON.stringify(site.background.light.trim())})` }
+        }
+        aria-hidden="true"
+      />
+      <div
+        className="site-background site-background-dark"
+        style={
+          site.background.dark.trim() === ''
+            ? undefined
+            : { backgroundImage: `url(${JSON.stringify(site.background.dark.trim())})` }
+        }
+        aria-hidden="true"
+      />
 
       <Layout
         contained

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

@@ -40,6 +40,7 @@ import {
   saveSettings,
   TONE_OPTIONS,
   type ProfileSettings,
+  type BackgroundSettings,
   type SettingsDocument,
   type SettingsTone,
   type SiteSettings,
@@ -63,6 +64,8 @@ import {
 import { createItem, deleteItem, fetchItem, saveItem } from '../../items'
 import { useProjects } from '../../useProjects'
 import type { Project } from '../../api'
+import siteBgLight from '../../assets/site-bg.jpg'
+import siteBgDark from '../../assets/site-bg-dark.jpg'
 
 /**
  * 新版后台：原子模型。
@@ -77,6 +80,7 @@ type TabKey =
   | 'home-sidebar'
   | 'home-cards'
   | 'home-header'
+  | 'home-background'
   | 'profile'
   | 'system'
 
@@ -89,6 +93,7 @@ const ADMIN_NAV: MenuItem[] = [
     label: '首页管理',
     children: [
       { key: 'home-header', label: '页头' },
+      { key: 'home-background', label: '背景管理' },
       { key: 'home-sidebar', label: '侧边栏' },
       { key: 'home-cards', label: '内容卡片' },
     ],
@@ -379,6 +384,13 @@ export function AdminNext() {
                   />
                 )}
 
+                {tab === 'home-background' && (
+                  <BackgroundPanel
+                    value={draft.background}
+                    onChange={(background) => setDraft({ ...draft, background })}
+                  />
+                )}
+
                 {tab === 'profile' && (
                   <ProfilePanel
                     value={draft.profile}
@@ -388,7 +400,9 @@ export function AdminNext() {
 
                 {tab === 'system' && <SystemPanel />}
 
-                {(tab === 'home-header' || tab === 'profile') && (
+                {(tab === 'home-header' ||
+                  tab === 'home-background' ||
+                  tab === 'profile') && (
                   <Space gap="sm" justify="end" className="admin-actions">
                     <Button
                       variant="secondary"
@@ -2037,6 +2051,64 @@ function HeaderPanel({
   )
 }
 
+function BackgroundPanel({
+  value,
+  onChange,
+}: {
+  value: BackgroundSettings
+  onChange: (background: BackgroundSettings) => void
+}) {
+  const update = (patch: Partial<BackgroundSettings>) => onChange({ ...value, ...patch })
+  const preview = (url: string, fallback: string) => ({
+    backgroundImage: `url(${JSON.stringify(url.trim() || fallback)})`,
+  })
+
+  return (
+    <Space direction="column" gap="lg" align="stretch" className="admin-form">
+      <Title level={4}>背景管理</Title>
+      <Text size="sm" tone="muted">
+        说明：分别设置亮色与暗色模式顶部的背景图，留空使用内置图；站内图片请放到托管目录 public/ 后填写路径，或填完整 URL。
+      </Text>
+
+      <div className="background-grid">
+        <div className="background-section">
+          <FormField label="亮色背景图" htmlFor="background-light" hint="留空使用内置亮色背景">
+            <Input
+              id="background-light"
+              value={value.light}
+              placeholder="/bg-light.jpg 或 https://…"
+              onChange={(event) => update({ light: event.target.value })}
+            />
+          </FormField>
+          <div className="background-preview" style={preview(value.light, siteBgLight)}>
+            <span>亮色预览</span>
+          </div>
+        </div>
+
+        <div className="background-section">
+          <FormField label="暗色背景图" htmlFor="background-dark" hint="留空使用内置暗色背景">
+            <Input
+              id="background-dark"
+              value={value.dark}
+              placeholder="/bg-dark.jpg 或 https://…"
+              onChange={(event) => update({ dark: event.target.value })}
+            />
+          </FormField>
+          <div className="background-preview" style={preview(value.dark, siteBgDark)}>
+            <span>暗色预览</span>
+          </div>
+        </div>
+      </div>
+
+      <div>
+        <Button variant="secondary" onClick={() => onChange({ light: '', dark: '' })}>
+          恢复内置背景
+        </Button>
+      </div>
+    </Space>
+  )
+}
+
 function ProfilePanel({
   value,
   onChange,

stash-web/src/settings.ts

@@ -23,6 +23,13 @@ export type ProfileSettings = {
   avatar: string
 }
 
+export type BackgroundSettings = {
+  /** 亮色背景图地址；空表示使用内置图 */
+  light: string
+  /** 暗色背景图地址；空表示使用内置图 */
+  dark: string
+}
+
 export type SiteSettings = {
   header: {
     enabled: boolean
@@ -40,6 +47,7 @@ export type SiteSettings = {
     /** 是否显示「分组导航」卡片（首页 / 分组树） */
     show_nav: boolean
   }
+  background: BackgroundSettings
   profile: ProfileSettings
   /** 站点级：标签标题 / favicon / 品牌 / 页头操作工具 */
   site: SiteMetaSettings
@@ -107,6 +115,7 @@ export function defaultSettings(): SiteSettings {
       show_service: true,
       show_nav: true,
     },
+    background: { light: '', dark: '' },
     profile: {
       name: '本地管理员',
       role: '管理员',