fix: 输出格式根据素材类型自动设置默认值

精灵/动画默认精灵表，背景/UI默认单帧，用户仍可手动切换。

frontend/src/stores/task.ts

@@ -28,6 +28,13 @@ interface TaskState {
   reset: () => void
 }
 
+const DEFAULT_FORMAT: Record<AssetType, 'spritesheet' | 'individual'> = {
+  sprite: 'spritesheet',
+  background: 'individual',
+  ui: 'individual',
+  animation: 'spritesheet',
+}
+
 const DEFAULT_PARAMS: TaskParams = {
   resolution: 1024,
   format: 'spritesheet',
@@ -44,7 +51,12 @@ export const useTaskStore = create<TaskState>((set, get) => ({
 
   setPrompt: (text) => set({ prompt: text, optimizedPrompt: null }),
 
-  setAssetType: (type) => set({ assetType: type, optimizedPrompt: null }),
+  setAssetType: (type) =>
+    set({
+      assetType: type,
+      params: { ...get().params, format: DEFAULT_FORMAT[type] },
+      optimizedPrompt: null,
+    }),
 
   toggleTaskStyle: (key, value) =>
     set(state => {