feat: implement multi-step background process for user registration

- Add step 1: Open ChatGPT homepage and prepare for signup.
- Add step 2: Handle email submission and ensure the signup entry page is ready.
- Add step 3: Fill in the password using either a custom or generated password.
- Add step 4: Confirm the registration verification page and handle email verification.
- Add step 5: Generate random user details and handle onboarding redirection.
- Add step 6: Manage OAuth login process and handle login verification.
- Add step 7: Retrieve verification code from email and manage retries.
- Add step 8: Handle consent page interactions and manage callback URLs.
- Add step 9: Submit callback information to CPA or SUB2API based on user settings.
- Implement tests to ensure all steps are correctly imported and functional.

background/steps/step3.js

@@ -0,0 +1,60 @@
+(function attachBackgroundStep3(root, factory) {
+  root.MultiPageBackgroundStep3 = factory();
+})(typeof self !== 'undefined' ? self : globalThis, function createBackgroundStep3Module() {
+  function createStep3Executor(deps = {}) {
+    const {
+      addLog,
+      chrome,
+      ensureContentScriptReadyOnTab,
+      generatePassword,
+      getTabId,
+      isTabAlive,
+      sendToContentScript,
+      setPasswordState,
+      setState,
+      SIGNUP_PAGE_INJECT_FILES,
+    } = deps;
+
+    async function executeStep3(state) {
+      const resolvedEmail = state.email;
+      if (!resolvedEmail) {
+        throw new Error('缺少邮箱地址，请先完成步骤 2。');
+      }
+
+      const signupTabId = await getTabId('signup-page');
+      if (!signupTabId || !(await isTabAlive('signup-page'))) {
+        throw new Error('认证页面标签页已关闭，请先重新完成步骤 2。');
+      }
+
+      const password = state.customPassword || generatePassword();
+      await setPasswordState(password);
+
+      const accounts = state.accounts || [];
+      accounts.push({ email: resolvedEmail, password, createdAt: new Date().toISOString() });
+      await setState({ accounts });
+
+      await chrome.tabs.update(signupTabId, { active: true });
+      await ensureContentScriptReadyOnTab('signup-page', signupTabId, {
+        inject: SIGNUP_PAGE_INJECT_FILES,
+        injectSource: 'signup-page',
+        timeoutMs: 45000,
+        retryDelayMs: 900,
+        logMessage: '步骤 3：密码页内容脚本未就绪，正在等待页面恢复...',
+      });
+
+      await addLog(
+        `步骤 3：正在填写密码，邮箱为 ${resolvedEmail}，密码为${state.customPassword ? '自定义' : '自动生成'}（${password.length} 位）`
+      );
+      await sendToContentScript('signup-page', {
+        type: 'EXECUTE_STEP',
+        step: 3,
+        source: 'background',
+        payload: { email: resolvedEmail, password },
+      });
+    }
+
+    return { executeStep3 };
+  }
+
+  return { createStep3Executor };
+});