fix(studio): hide sidebar group until token resolves; remove VITE_STUDIO_ENABLED

- workspace-nav-studio: return null while isLoading so the "工作室" group
  no longer briefly mounts and then unmounts when getTokenByEmail returns
  "用户不存在" (404). Skeleton branch became dead code, removed.
- studio/hooks: switch all three token queries from retry: 1 to false —
  account-not-found is a permanent error; retry just doubled isError latency.
- env: drop VITE_STUDIO_ENABLED everywhere (env.ts + 7 env files +
  workspace-nav-chat-list). Sidebar availability is now decided at runtime
  by token success instead of a build-time flag.
- env: add VITE_AIPI_PRESENT_URL + VITE_AIPI_KNOWES_URL for the document
  upload popup's A-system jump + B-system knowledge iframe; reuse the
  existing VITE_AIPI_KNOWLEDGE_URL for the Y-system iframe.
- document-upload-popup: replace 8 hardcoded URLs (6 present, 1 knowes,
  1 knowlibrary) with the corresponding env vars.

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

frontend-web/.env.example

@@ -23,7 +23,6 @@
 # VITE_NOTE_API_BASE_URL=http://47.88.25.99:8000
 # VITE_AIPI_BACKENP_URL=https://knowledgetoolserver.b.uat.4.cn
 # VITE_AIPI_FRONTEND_URL=https://knowledgetoolserver.b.uat.4.cn
-# VITE_STUDIO_ENABLED=true  # 仅控制“工作室”入口显示，列表内容仍依赖 notebook 接口数据
 
 # --- 生产打包（见 .env.production）---
 # 部署前修改 .env.production 中的地址，再执行 pnpm build