feat(search): 按参考稿重做搜索框并支持后台配置

- 顶部悬浮引擎标签 + 下拉选项，使用 stash-ui 新增的 EngineIcon
- 站内搜索保持现有本地过滤与回车打开首个匹配项
- 外部引擎使用配置的 {query} 模板新标签页搜索
- settings 新增 search.default / search.engines，后台新增『搜索框』配置默认项、名称、地址与启停
- 默认默认搜索项为站内搜索，地址校验必须含 {query}

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

@@ -4,6 +4,7 @@ import {
   Card,
   Caret,
   Empty,
+  EngineIcon,
   FormField,
   Icon,
   ICON_NAMES,
@@ -27,6 +28,7 @@ import {
   ToggleRow,
   Toaster,
   toast,
+  type EngineIconName,
   type IconName,
   type MenuItem,
 } from '@stash/ui'
@@ -36,11 +38,13 @@ import { LoginDialog } from '../auth/LoginDialog'
 import { GroupMark as GroupGlyph } from '../common/GroupMark'
 import { isDarkScheme, readThemeOverride, writeThemeOverride } from '../../theme'
 import {
+  DEFAULT_SEARCH_ENGINES,
   fetchSettings,
   saveSettings,
   TONE_OPTIONS,
-  type ProfileSettings,
   type BackgroundSettings,
+  type ProfileSettings,
+  type SearchSettings,
   type SettingsDocument,
   type SettingsTone,
   type SiteSettings,
@@ -81,6 +85,7 @@ type TabKey =
   | 'home-cards'
   | 'home-header'
   | 'home-background'
+  | 'home-search'
   | 'profile'
   | 'system'
 
@@ -94,6 +99,7 @@ const ADMIN_NAV: MenuItem[] = [
     children: [
       { key: 'home-header', label: '页头' },
       { key: 'home-background', label: '背景管理' },
+      { key: 'home-search', label: '搜索框' },
       { key: 'home-sidebar', label: '侧边栏' },
       { key: 'home-cards', label: '内容卡片' },
     ],
@@ -391,6 +397,13 @@ export function AdminNext() {
                   />
                 )}
 
+                {tab === 'home-search' && (
+                  <SearchPanel
+                    value={draft.search}
+                    onChange={(search) => setDraft({ ...draft, search })}
+                  />
+                )}
+
                 {tab === 'profile' && (
                   <ProfilePanel
                     value={draft.profile}
@@ -402,6 +415,7 @@ export function AdminNext() {
 
                 {(tab === 'home-header' ||
                   tab === 'home-background' ||
+                  tab === 'home-search' ||
                   tab === 'profile') && (
                   <Space gap="sm" justify="end" className="admin-actions">
                     <Button
@@ -2051,6 +2065,104 @@ function HeaderPanel({
   )
 }
 
+function SearchPanel({
+  value,
+  onChange,
+}: {
+  value: SearchSettings
+  onChange: (search: SearchSettings) => void
+}) {
+  const updateEngine = (
+    id: string,
+    patch: Partial<SearchSettings['engines'][number]>,
+  ) => {
+    const engines = value.engines.map((engine) =>
+      engine.id === id ? { ...engine, ...patch } : engine,
+    )
+    const nextDefault =
+      patch.enabled === false && value.default === id ? 'internal' : value.default
+    onChange({ ...value, engines, default: nextDefault })
+  }
+
+  const defaultOptions = [
+    { value: 'internal', label: '站内搜索' },
+    ...value.engines
+      .filter((engine) => engine.enabled)
+      .map((engine) => ({ value: engine.id, label: engine.name })),
+  ]
+
+  return (
+    <Space direction="column" gap="lg" align="stretch" className="admin-form">
+      <Title level={4}>搜索框</Title>
+      <Text size="sm" tone="muted">
+        说明：站内搜索始终可用；外部搜索引擎可以改名称、地址和启停，地址里的 {'{query}'}{' '}
+        会替换成关键词。默认项是刷新后搜索框选中的引擎。
+      </Text>
+
+      <FormField label="默认搜索项">
+        <Select
+          options={defaultOptions}
+          value={value.default}
+          onChange={(next) => onChange({ ...value, default: next })}
+        />
+      </FormField>
+
+      <div className="search-engine-list">
+        {value.engines.map((engine) => (
+          <div
+            className={`search-engine-row${engine.enabled ? '' : ' is-disabled'}`}
+            key={engine.id}
+          >
+            <div className="search-engine-head">
+              <EngineIcon name={engine.id as EngineIconName} size={18} />
+              <Text strong>{engine.name || engine.id}</Text>
+              <Switch
+                checked={engine.enabled}
+                label={`启用 ${engine.name || engine.id}`}
+                onChange={(enabled) => updateEngine(engine.id, { enabled })}
+              />
+            </div>
+            <div className="search-engine-fields">
+              <FormField label="名称" htmlFor={`search-name-${engine.id}`}>
+                <Input
+                  id={`search-name-${engine.id}`}
+                  value={engine.name}
+                  onChange={(event) => updateEngine(engine.id, { name: event.target.value })}
+                />
+              </FormField>
+              <FormField
+                label="搜索地址"
+                htmlFor={`search-url-${engine.id}`}
+                hint="必须包含 {query}"
+              >
+                <Input
+                  id={`search-url-${engine.id}`}
+                  value={engine.url}
+                  onChange={(event) => updateEngine(engine.id, { url: event.target.value })}
+                />
+              </FormField>
+            </div>
+          </div>
+        ))}
+      </div>
+
+      <div>
+        <Button
+          variant="secondary"
+          onClick={() =>
+            onChange({
+              default: 'internal',
+              engines: DEFAULT_SEARCH_ENGINES.map((engine) => ({ ...engine })),
+            })
+          }
+        >
+          恢复默认引擎
+        </Button>
+      </div>
+    </Space>
+  )
+}
+
 function BackgroundPanel({
   value,
   onChange,