refactor: favicon 移回 app/ + 删除用户改为 RESTful DELETE /api/users/[id]

- favicon: 从 public/favicon.ico 移回 app/favicon.ico（next-on-pages 1.13.16 下生产验证 /favicon.ico 正常返回）
- 删除用户: POST /api/roles/delete 改为 DELETE /api/users/[id]，符合 RESTful 语义（资源 users + 标准 DELETE 方法）

app/components/profile/promote-panel.tsx

@@ -135,10 +135,8 @@ export function PromotePanel() {
   const handleDelete = async (user: UserItem) => {
     setDeletingUserId(user.id)
     try {
-      const res = await fetch("/api/roles/delete", {
-        method: "POST",
-        headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ userId: user.id }),
+      const res = await fetch(`/api/users/${user.id}`, {
+        method: "DELETE",
       })
       if (!res.ok) {
         const error = await res.json() as { error: string }