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.production.local.example

@@ -5,4 +5,3 @@
 # VITE_NOTE_API_BASE_URL=https://your-intranet-host/note-api
 # VITE_AIPI_BACKENP_URL=https://your-notebook-backend-host
 # VITE_AIPI_FRONTEND_URL=https://your-notebook-frontend-host
-# VITE_STUDIO_ENABLED=true