feat(api): real login + listening wired to the backend contract

Replace the faked ConnectPage login with a real /auth/login -> /auth/me
flow, including loading/error states. Add a backend-contract adapter layer
(api/mappers.ts) translating the backend's snake_case, lean *Out schemas and
{items,total,limit,offset} paging into the UI's camelCase domain types, so
swapping backends only touches the mappers.

- auth: chained login (tokens) + /auth/me (user); refresh on snake_case;
  expiresIn optional (reauth is 401-driven, backend sends no TTL)
- streaming: GET /stream/{id}?token= (token query param for <audio>); SW
  audio cache route + tests follow the path change (token stays cache-stable)
- library/playlists/likes/admin: correct paths (/tracks not /library/tracks),
  page/pageSize<->limit/offset, duration_seconds->durationMs, likes as
  append-only POST /likes event-log, admin is_superuser<->role
- downloads/storage: marked provisional (backend routes still stubs)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/endpoints/downloads.ts

@@ -1,6 +1,12 @@
 import { api } from '../index';
 import type { DownloadJob } from '../types';
 
+// NOTE: the backend `/downloads` routes are still unimplemented stubs (they
+// return no body / no schema). The request shapes below are provisional and the
+// responses will need the same snake→camel mapper treatment as library/playlists
+// (see `mappers.ts`) once the backend defines DownloadJob's wire format. Do not
+// wire these into the UI until then.
+
 export const downloadsApi = api.injectEndpoints({
   endpoints: (build) => ({
     getDownloads: build.query<