feat: refactor user registration steps into modular functions and implement step registry

- Replaced individual step imports with a centralized step registry in background.js
- Created separate modules for each registration step: open-chatgpt, submit-signup-email, fill-password, fetch-signup-code, fill-profile, oauth-login, fetch-login-code, confirm-oauth, platform-verify
- Updated tests to reflect changes in step imports and added new tests for step registry functionality

background/steps/open-chatgpt.js

@@ -0,0 +1,21 @@
+(function attachBackgroundStep1(root, factory) {
+  root.MultiPageBackgroundStep1 = factory();
+})(typeof self !== 'undefined' ? self : globalThis, function createBackgroundStep1Module() {
+  function createStep1Executor(deps = {}) {
+    const {
+      addLog,
+      completeStepFromBackground,
+      ensureSignupEntryPageReady,
+    } = deps;
+
+    async function executeStep1() {
+      await addLog('步骤 1：正在打开 ChatGPT 官网...');
+      await ensureSignupEntryPageReady(1);
+      await completeStepFromBackground(1, {});
+    }
+
+    return { executeStep1 };
+  }
+
+  return { createStep1Executor };
+});