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/src/env.ts

@@ -9,8 +9,7 @@ function readEnv(
     | "VITE_AIPI_BACKENP_URL"
     | "VITE_AIPI_FRONTEND_URL"
     | "VITE_STATIC_WEBSITE_ONLY"
-    | "VITE_GITHUB_OAUTH_TOKEN"
-    | "VITE_STUDIO_ENABLED",
+    | "VITE_GITHUB_OAUTH_TOKEN",
 ): EnvValue {
   const value = import.meta.env[key];
   return typeof value === "string" && value.trim() ? value : undefined;
@@ -26,5 +25,4 @@ export const env = {
   NEXT_PUBLIC_NOTEBOOK_API_BASE_URL: readEnv("VITE_AIPI_BACKENP_URL"),
   NEXT_PUBLIC_STATIC_WEBSITE_ONLY: readEnv("VITE_STATIC_WEBSITE_ONLY"),
   GITHUB_OAUTH_TOKEN: readEnv("VITE_GITHUB_OAUTH_TOKEN"),
-  STUDIO_ENABLED: readEnv("VITE_STUDIO_ENABLED"),
 };