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/streaming.ts

@@ -1,8 +1,13 @@
 import { getApiBaseUrl } from '../../config/runtime-config';
 
+/**
+ * Audio stream URL for the `<audio>` element. The access token rides as a query
+ * param because `<audio>` can't send an `Authorization` header; the backend
+ * accepts `?token=` on `GET /stream/{id}` for exactly this reason.
+ */
 export function getStreamUrl(trackId: string, token: string): string {
   const base = getApiBaseUrl();
-  return `${base}/streaming/tracks/${trackId}?token=${encodeURIComponent(token)}`;
+  return `${base}/stream/${trackId}?token=${encodeURIComponent(token)}`;
 }
 
 export function getCoverUrl(artUrl: string | undefined): string | undefined {