🐛 fix(table-designer): 修复 DuckDB 表设计主键保存失效

- 为 DuckDB 表结构变更补充 ADD PRIMARY KEY 预览 SQL
- 保存前拦截已有主键表的主键替换与删除，避免假成功
- 补充 DuckDB 主键变更判定与 schema SQL 回归测试

frontend/src/components/tableDesignerDuckDbPrimaryKey.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+
+import type { EditableColumnSnapshot } from './tableDesignerSchemaSql';
+import { summarizeDuckDbPrimaryKeyChange } from './tableDesignerDuckDbPrimaryKey';
+
+const col = (overrides: Partial<EditableColumnSnapshot>): EditableColumnSnapshot => ({
+  _key: overrides._key || 'id',
+  name: overrides.name || 'id',
+  type: overrides.type || 'BIGINT',
+  nullable: overrides.nullable || 'NO',
+  default: overrides.default || '',
+  extra: overrides.extra || '',
+  comment: overrides.comment || '',
+  key: overrides.key || '',
+  isAutoIncrement: overrides.isAutoIncrement || false,
+});
+
+describe('tableDesignerDuckDbPrimaryKey', () => {
+  it('treats first primary key addition as supported', () => {
+    const summary = summarizeDuckDbPrimaryKeyChange(
+      [col({ _key: 'id', key: '' })],
+      [col({ _key: 'id', key: 'PRI' })],
+    );
+
+    expect(summary).toEqual({
+      hasChange: true,
+      isAddingPrimaryKey: true,
+      isUnsupportedChange: false,
+    });
+  });
+
+  it('treats replacing an existing primary key as unsupported', () => {
+    const summary = summarizeDuckDbPrimaryKeyChange(
+      [col({ _key: 'id', key: 'PRI' }), col({ _key: 'name', name: 'name', key: '' })],
+      [col({ _key: 'id', key: '' }), col({ _key: 'name', name: 'name', key: 'PRI' })],
+    );
+
+    expect(summary).toEqual({
+      hasChange: true,
+      isAddingPrimaryKey: false,
+      isUnsupportedChange: true,
+    });
+  });
+});