feat: add platform type filter to subscription management page

Add a platform filter dropdown to the admin subscriptions view, allowing
filtering subscriptions by platform (Anthropic, OpenAI, Gemini, etc.)
through the group association.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -1694,6 +1694,7 @@ export default {
       revokeSubscription: 'Revoke Subscription',
       allStatus: 'All Status',
       allGroups: 'All Groups',
+      allPlatforms: 'All Platforms',
       daily: 'Daily',
       weekly: 'Weekly',
       monthly: 'Monthly',

frontend/src/i18n/locales/zh.ts

@@ -1774,6 +1774,7 @@ export default {
       revokeSubscription: '撤销订阅',
       allStatus: '全部状态',
       allGroups: '全部分组',
+      allPlatforms: '全部平台',
       daily: '每日',
       weekly: '每周',
       monthly: '每月',