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>

public/sw-core.js

@@ -19,8 +19,8 @@ export const MAX_BYTES = 500 * 1024 * 1024; // 500 MB
 export const COVER_CACHE = 'mcma-covers-v1';
 export const MAX_COVERS = 600;
 
-// Backend stream route: /api/v1/streaming/tracks/<trackId>?token=...
-const STREAM_RE = /\/streaming\/tracks\/([^/?#]+)/;
+// Backend stream route: /api/v1/stream/<trackId>?token=...
+const STREAM_RE = /\/stream\/([^/?#]+)/;
 
 /** The track (content) id from a stream URL, or null if it isn't one. */
 export function trackIdFromUrl(url) {