feat(search): 内置 DuckDuckGo，内置引擎不允许删除

- 默认引擎清单加 duckduckgo（https://duckduckgo.com/?q={query}），图标用组件库 0.3.5 的 DuckDuckGo 品牌图标
- 后端：内置六个必须存在，缺了返回 400；老库读取时按 id 补齐缺的内置引擎（插在内置那一组里，不覆盖用户改过的值）
- 后台：内置引擎不渲染「删除」按钮，removeEngine 也做了兜底；自定义引擎照常可删

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

@@ -37,6 +37,7 @@ import { LoginDialog } from '../auth/LoginDialog'
 import { GroupMark as GroupGlyph } from '../common/GroupMark'
 import { isDarkScheme, readThemeOverride, writeThemeOverride } from '../../theme'
 import {
+  BUILTIN_SEARCH_ENGINE_IDS,
   DEFAULT_SEARCH_ENGINES,
   fetchSettings,
   saveSettings,
@@ -2110,6 +2111,8 @@ function SearchPanel({
   }
 
   const removeEngine = (id: string) => {
+    // 内置引擎只能停用，不能删
+    if (BUILTIN_SEARCH_ENGINE_IDS.some((builtin) => builtin === id)) return
     const engines = value.engines.filter((engine) => engine.id !== id)
     onChange({
       ...value,
@@ -2156,7 +2159,9 @@ function SearchPanel({
                   label={`启用 ${engine.name || engine.id}`}
                   onChange={(enabled) => updateEngine(engine.id, { enabled })}
                 />
-                <TextButton onClick={() => removeEngine(engine.id)}>删除</TextButton>
+                {!BUILTIN_SEARCH_ENGINE_IDS.some((builtin) => builtin === engine.id) && (
+                  <TextButton onClick={() => removeEngine(engine.id)}>删除</TextButton>
+                )}
               </div>
             </div>
             <div className="search-engine-fields">

stash-web/src/settings.ts

@@ -132,8 +132,25 @@ export const DEFAULT_SEARCH_ENGINES: SearchEngineSettings[] = [
     icon: '',
     enabled: true,
   },
+  {
+    id: 'duckduckgo',
+    name: 'DuckDuckGo',
+    url: 'https://duckduckgo.com/?q={query}',
+    icon: '',
+    enabled: true,
+  },
 ]
 
+/** 内置引擎：有品牌图标、后台不能删除（只能改名 / 改地址 / 停用） */
+export const BUILTIN_SEARCH_ENGINE_IDS = [
+  'google',
+  'bing',
+  'baidu',
+  'sogou',
+  '360',
+  'duckduckgo',
+] as const
+
 export type SiteSettings = {
   header: {
     enabled: boolean