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

tests/background-step-modules.test.js

@@ -6,15 +6,15 @@ test('background imports step 1~9 modules', () => {
   const source = fs.readFileSync('background.js', 'utf8');
 
   [
-    'background/steps/step1.js',
-    'background/steps/step2.js',
-    'background/steps/step3.js',
-    'background/steps/step4.js',
-    'background/steps/step5.js',
-    'background/steps/step6.js',
-    'background/steps/step7.js',
-    'background/steps/step8.js',
-    'background/steps/step9.js',
+    'background/steps/open-chatgpt.js',
+    'background/steps/submit-signup-email.js',
+    'background/steps/fill-password.js',
+    'background/steps/fetch-signup-code.js',
+    'background/steps/fill-profile.js',
+    'background/steps/oauth-login.js',
+    'background/steps/fetch-login-code.js',
+    'background/steps/confirm-oauth.js',
+    'background/steps/platform-verify.js',
   ].forEach((path) => {
     assert.match(source, new RegExp(path.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')));
   });