chore: make prettier 自动清理前后端无用 import

前端接入 eslint-plugin-unused-imports，pnpm format 同步执行 eslint --fix；
后端将 gofmt 替换为 goimports，并修复 danger-zone 未使用导入。

frontend/eslint.config.mjs

@@ -1,6 +1,7 @@
 import { dirname } from 'path';
 import { fileURLToPath } from 'url';
 import { FlatCompat } from '@eslint/eslintrc';
+import unusedImports from 'eslint-plugin-unused-imports';
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = dirname(__filename);
@@ -20,6 +21,25 @@ const eslintConfig = [
       'next-env.d.ts',
     ],
   },
+  {
+    plugins: {
+      'unused-imports': unusedImports,
+    },
+    rules: {
+      // Prefer unused-imports so unused imports can be auto-removed with --fix.
+      '@typescript-eslint/no-unused-vars': 'off',
+      'unused-imports/no-unused-imports': 'error',
+      'unused-imports/no-unused-vars': [
+        'warn',
+        {
+          vars: 'all',
+          varsIgnorePattern: '^_',
+          args: 'after-used',
+          argsIgnorePattern: '^_',
+        },
+      ],
+    },
+  },
 ];
 
 export default eslintConfig;