feat: 分组图标统一（iconfont 名字优先，字符兜底）+ 后台选图标

- 新增 components/common/GroupMark：值是图标名就渲染图标，否则按字符显示，
  兼容旧数据里的 emoji；后台的 GroupMark 改为复用同一判断
- 分组详情页 hero 之前把图标名当文本渲染（会直接显示 folder_2_line），已修
- 分组编辑弹窗加「选择图标」：搜索 + 网格，446 个图标按关键字过滤（默认展示前 120）
- 输入框仍可手填字符，旧数据不用迁移

stash-web/src/AppNext.tsx

@@ -18,6 +18,7 @@ import { ItemEditor } from './components/content/ItemEditor'
 import { PosterGrid } from './components/content/PosterGrid'
 import { SectionCard } from './components/content/SectionCard'
 import { SiteHeader } from './components/header/SiteHeader'
+import { GroupMark } from './components/common/GroupMark'
 import { LoginDialog } from './components/auth/LoginDialog'
 import { ProfileCard } from './components/sidebar/ProfileCard'
 import { ServicePanel } from './components/sidebar/ServicePanel'
@@ -486,7 +487,7 @@ export default function AppNext() {
 
         <div className="detail-hero">
           <span className="detail-hero-icon" aria-hidden="true">
-            {group.icon || '•'}
+            <GroupMark icon={group.icon} fallback="•" />
           </span>
           <h2 className="detail-hero-title">{group.title}</h2>
           <span className="detail-hero-count">[{groupEntryCount(group)}]</span>