fix: 代码清理和模型名称修正

- 移除未使用的showToast导入
- 恢复正确的模型名称为gemini-2.5-flash-image-preview

src/pages/index/index.tsx

@@ -1,5 +1,5 @@
 import { View, Text, Button, Image } from '@tarojs/components'
-import { useLoad, showToast } from '@tarojs/taro'
+import { useLoad } from '@tarojs/taro'
 import { useState } from 'react'
 import './index.css'
 import { useSdk } from '../../hooks/index'

src/sdk/index.ts

@@ -229,7 +229,7 @@ export class BowongAISDK {
   async generateImage(params: GenerateImageParams): Promise<string> {
     const {
       prompt,
-      model_name = 'gemini-2.5-flash-image',
+      model_name = 'gemini-2.5-flash-image-preview',
       aspect_ratio = '9:16',
       mode = 'turbo',
       webhook_flag = false,