fix: PNG cover 404 via nginx + clear upload residual after submit

- Fix UAT nginx static cache rule intercepting /api/files/*.png requests,
  use negative lookahead to exclude /api/ paths (same as Dev config)
- Clear el-upload file list after successful game creation to prevent
  residual file path showing in next upload

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/game/AddGameDialog.vue

@@ -5,7 +5,7 @@ import type { GamePlatform } from '@/types'
 import { ElMessage } from 'element-plus'
 import { getAllPlatforms } from '@/api/games'
 import { Plus } from '@element-plus/icons-vue'
-import type { UploadFile } from 'element-plus'
+import type { UploadFile, UploadInstance } from 'element-plus'
 
 const props = defineProps<{
   modelValue: boolean
@@ -27,6 +27,7 @@ const platform = ref<GamePlatform | ''>('')
 const tagsInput = ref('')
 const coverFile = ref<File | null>(null)
 const coverPreview = ref('')
+const uploadRef = ref<UploadInstance>()
 const loading = ref(false)
 const platforms = getAllPlatforms()
 
@@ -62,6 +63,7 @@ async function handleSubmit() {
     tagsInput.value = ''
     coverFile.value = null
     coverPreview.value = ''
+    uploadRef.value?.clearFiles()
     emit('created')
     visible.value = false
   } catch (error: any) {
@@ -92,6 +94,7 @@ async function handleSubmit() {
       <div class="field">
         <label>封面图</label>
         <el-upload
+          ref="uploadRef"
           :auto-upload="false"
           :limit="1"
           accept="image/*"