feat: support custom email pool in the merged dev registration flow

- 吸收 dev 最新 sidepanel 与收码链路改动，保留 Cloudflare Temp Email 独立设置区、贡献模式自动前提醒和 2925 当前账号恢复接线

- 修复生成邮箱辅助层对运行态参数的判断，避免 Gmail 别名与自定义邮箱池误用旧的已保存状态

- 补充自定义邮箱池相关测试与文档，更新 README、项目结构说明和完整链路说明

background/generated-email-helpers.js

@@ -148,6 +148,7 @@
       const requestedName = String(options.localPart || options.name || '').trim().toLowerCase() || generateCloudflareAliasLocalPart();
       const payload = {
         enablePrefix: true,
+        enableRandomSubdomain: Boolean(config.useRandomSubdomain),
         name: requestedName,
         domain: config.domain,
       };
@@ -254,26 +255,41 @@
         ? options.mail2925Mode
         : currentState.mail2925Mode;
       const generator = normalizeEmailGenerator(options.generator ?? currentState.emailGenerator);
+      const mergedState = {
+        ...currentState,
+        mailProvider: provider || currentState.mailProvider,
+        mail2925Mode,
+        emailGenerator: generator,
+      };
+      if (options.gmailBaseEmail !== undefined) {
+        mergedState.gmailBaseEmail = String(options.gmailBaseEmail || '').trim();
+      }
+      if (options.mail2925BaseEmail !== undefined) {
+        mergedState.mail2925BaseEmail = String(options.mail2925BaseEmail || '').trim();
+      }
+      if (options.customEmailPool !== undefined) {
+        mergedState.customEmailPool = options.customEmailPool;
+      }
       if (generator === 'custom') {
         throw new Error('当前邮箱生成方式为自定义邮箱，请直接填写注册邮箱。');
       }
       if (generator === CUSTOM_EMAIL_POOL_GENERATOR) {
-        return fetchCustomEmailPoolEmail(currentState, options);
+        return fetchCustomEmailPoolEmail(mergedState, options);
       }
       const shouldUseManagedAlias = typeof isGeneratedAliasProvider === 'function'
-        ? isGeneratedAliasProvider(currentState, mail2925Mode)
+        ? isGeneratedAliasProvider(mergedState, mail2925Mode)
         : false;
       if (shouldUseManagedAlias) {
-        return fetchManagedAliasEmail(currentState, options);
+        return fetchManagedAliasEmail(mergedState, options);
       }
       if (generator === 'icloud') {
         return fetchIcloudHideMyEmail();
       }
       if (generator === 'cloudflare') {
-        return fetchCloudflareEmail(currentState, options);
+        return fetchCloudflareEmail(mergedState, options);
       }
       if (generator === CLOUDFLARE_TEMP_EMAIL_GENERATOR) {
-        return fetchCloudflareTempEmailAddress(currentState, options);
+        return fetchCloudflareTempEmailAddress(mergedState, options);
       }
       return fetchDuckEmail(options);
     }