feat(workspace): replace supported platform count with bound media account count

The "platforms" stat tile was a constant (5) that never reflected what the
tenant actually had set up. Drop SupportedPlatformCount from the workspace
overview API/domain/shared-types and have admin-web compute a bound media
account count from the desktop accounts list, filtered through a new
isMediaPublishAccount helper that excludes deleted rows and AI-only
platforms (yuanbao/kimi/wenxin/deepseek/doubao/qwen).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/admin-web/src/i18n/messages/en-US.ts

@@ -379,7 +379,7 @@ const enUS = {
       articles: "Generated",
       published: "Published",
       brands: "Brands",
-      platforms: "Platforms",
+      mediaAccounts: "Media accounts",
     },
     sections: {
       generalTemplates: "General-Purpose Templates",

apps/admin-web/src/i18n/messages/zh-CN.ts

@@ -380,7 +380,7 @@ const zhCN = {
       articles: "生成数",
       published: "发布数",
       brands: "品牌数",
-      platforms: "媒体平台数",
+      mediaAccounts: "媒体账号数",
     },
     sections: {
       generalTemplates: "普通通用模版",