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/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,