🐛 fix(sql-snippet): 修复片段管理编辑与按钮布局

- 支持自定义 SQL 片段按 id 更新，避免修改时重复新增
- 将片段语法说明改为可编辑并随片段持久化
- 将保存、删除、重置、关闭按钮统一收敛到底部操作栏
- 调整操作按钮为大号尺寸并增加最小宽度和底部间距
- 补充片段编辑、布局结构和持久化回归测试

frontend/src/components/QueryEditor.external-sql-save.test.tsx

@@ -4367,7 +4367,14 @@ describe('QueryEditor external SQL save', () => {
     const modalSource = readFileSync(new URL('./SnippetSettingsModal.tsx', import.meta.url), 'utf8');
     const source = readFileSync(new URL('./QueryEditor.tsx', import.meta.url), 'utf8');
 
-    expect(modalSource).toContain('片段语法说明（可选）');
+    expect(modalSource).toContain('片段语法说明（可编辑）');
+    expect(modalSource).toContain('data-sql-snippet-syntax-help-editor="true"');
+    expect(modalSource).toContain("defaultActiveKey={['snippet-help']}");
+    expect(modalSource).toContain('footer={null}');
+    expect(modalSource).toContain('data-sql-snippet-action-row="true"');
+    expect(modalSource).toContain('body: { paddingTop: 8, paddingBottom: 24 }');
+    expect(modalSource).toContain("size=\"large\"");
+    expect(modalSource).toContain('minWidth: 96');
     expect(modalSource).toContain('syntaxHelp');
     expect(modalSource).toContain('占位符语法参考');
     expect(source).toContain('s.syntaxHelp || s.description || s.body');