feat: 建立可扩展网页后端抽象

拆分后端定义、注册表、DOM 适配器和 sandbox preload，统一千问与元宝的就绪、登录和完整文本上报契约。

AI-assisted-by: Codex
AI-model: gpt-5.6-sol
AI-reasoning-effort: medium
Co-authored-by: Codex <codex@openai.com>

test/backends.test.ts

@@ -0,0 +1,20 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { backendOwnsUrl } from '../src/backends/contracts';
+import { getBackend, listBackends, requireBackend } from '../src/backends/registry';
+
+test('后端注册表包含千问和元宝且 ID 唯一', () => {
+  const backends = listBackends();
+  assert.deepEqual(backends.map(({ id }) => id), ['qianwen', 'yuanbao']);
+  assert.equal(new Set(backends.map(({ id }) => id)).size, backends.length);
+  assert.equal(getBackend('qianwen')?.label, '千问');
+  assert.equal(getBackend('missing'), undefined);
+  assert.throws(() => requireBackend('missing'), /未知后端/);
+});
+
+test('后端 URL 校验使用精确 origin', () => {
+  const qianwen = requireBackend('qianwen');
+  assert.equal(backendOwnsUrl(qianwen, 'https://www.qianwen.com/chat/1'), true);
+  assert.equal(backendOwnsUrl(qianwen, 'https://www.qianwen.com.evil.example/'), false);
+  assert.equal(backendOwnsUrl(qianwen, 'not a url'), false);
+});