First step of splitting the api.ts god module behind its façade. Move req,
HttpError, localizeDetail, the connectivity toast, ReqConfig, the per-tab
active-account plumbing (getActiveAccount/setActiveAccount/clearActiveAccount/
beacon), setupHeaders, and setUnauthorizedHandler into lib/api/core.ts. api.ts
imports the machinery it uses and re-exports the public surface, so the 53 call
sites importing from "./api" are unchanged. api.ts 1793 -> 1583 lines; no
runtime change (pure move). Gate green: typecheck, eslint, prettier, 74 vitest.