fix: 修复 CategoryFormDialog TypeScript 类型收窄导致的构建错误

将 category?.slug 提取为独立变量 currentSlug，避免 TypeScript
在 if (!isEditing) 分支内将 category 收窄为 never 类型。

src/components/category/CategoryFormDialog.tsx

@@ -72,6 +72,7 @@ export function CategoryFormDialog({
   onSubmit,
 }: CategoryFormDialogProps) {
   const isEditing = !!category
+  const currentSlug = category?.slug
 
   const {
     register,
@@ -126,7 +127,7 @@ export function CategoryFormDialog({
               {...register("name", {
                 onChange: (e: React.ChangeEvent<HTMLInputElement>) => {
                   if (!isEditing) {
-                    setValue("slug", generateUniqueSlug(e.target.value, existingSlugs, category?.slug), {
+                    setValue("slug", generateUniqueSlug(e.target.value, existingSlugs, currentSlug), {
                       shouldValidate: true,
                     })
                   }