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/registry.js

@@ -0,0 +1,48 @@
+(function attachBackgroundStepRegistry(root, factory) {
+  root.MultiPageBackgroundStepRegistry = factory();
+})(typeof self !== 'undefined' ? self : globalThis, function createBackgroundStepRegistryModule() {
+  function createStepRegistry(definitions = []) {
+    const ordered = (Array.isArray(definitions) ? definitions : [])
+      .map((definition) => ({
+        id: Number(definition?.id),
+        order: Number(definition?.order),
+        key: String(definition?.key || '').trim(),
+        title: String(definition?.title || '').trim(),
+        execute: definition?.execute,
+      }))
+      .filter((definition) => Number.isFinite(definition.id) && typeof definition.execute === 'function')
+      .sort((left, right) => {
+        const leftOrder = Number.isFinite(left.order) ? left.order : left.id;
+        const rightOrder = Number.isFinite(right.order) ? right.order : right.id;
+        return leftOrder - rightOrder;
+      });
+
+    const byId = new Map(ordered.map((definition) => [definition.id, definition]));
+
+    function getStepDefinition(step) {
+      return byId.get(Number(step)) || null;
+    }
+
+    function getOrderedSteps() {
+      return ordered.slice();
+    }
+
+    function executeStep(step, state) {
+      const definition = getStepDefinition(step);
+      if (!definition) {
+        throw new Error(`未知步骤：${step}`);
+      }
+      return definition.execute(state);
+    }
+
+    return {
+      executeStep,
+      getOrderedSteps,
+      getStepDefinition,
+    };
+  }
+
+  return {
+    createStepRegistry,
+  };
+});