feat(search): 自定义引擎支持图标地址（png / svg）

- settings.search.engines[] 新增 icon：空=品牌图标/通用地球，'/' 开头走托管目录，或 http(s) 链接
- 后端校验图标地址（空 / 托管路径 / http(s)，≤500 字符）
- 后台每张引擎卡加「图标地址」输入；前后台都用组件库 EngineIcon 的 src（<img> 渲染，SVG 静态不执行脚本）
- @stash/ui 0.3.4

stash-web/src/AppNext.tsx

@@ -69,7 +69,7 @@ type Route =
   | { kind: 'item'; key: string; itemId: string }
   | { kind: 'api' }
 
-const INTERNAL_SEARCH = { id: 'internal', name: '站内搜索', url: '' } as const
+const INTERNAL_SEARCH = { id: 'internal', name: '站内搜索', url: '', icon: '' } as const
 
 const SEARCH_ENGINE_KEY = 'stash-search-engine'
 
@@ -362,7 +362,12 @@ export default function AppNext() {
     INTERNAL_SEARCH,
     ...site.search.engines
       .filter((engine) => engine.enabled)
-      .map((engine) => ({ id: engine.id, name: engine.name, url: engine.url })),
+      .map((engine) => ({
+        id: engine.id,
+        name: engine.name,
+        url: engine.url,
+        icon: engine.icon ?? '',
+      })),
   ]
   const searchEngine =
     searchEngines.find((engine) => engine.id === searchEngineId) ??
@@ -930,7 +935,7 @@ export default function AppNext() {
                 }
                 onClick={() => setEngineOpen((open) => !open)}
               >
-                <EngineIcon name={searchEngine.id} size={14} />
+                <EngineIcon name={searchEngine.id} src={searchEngine.icon} size={14} />
                 <span>{searchEngine.name}</span>
               </button>
 
@@ -977,7 +982,7 @@ export default function AppNext() {
                           className={`engine-option${engine.id === searchEngine.id ? ' is-active' : ''}`}
                           onClick={() => chooseEngine(engine.id)}
                         >
-                          <EngineIcon name={engine.id} size={18} />
+                          <EngineIcon name={engine.id} src={engine.icon} size={18} />
                           <span>{engine.name}</span>
                         </button>
                       ))}