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/i18n/locales/en.ts

@@ -35,8 +35,12 @@ const en = {
       username: 'Username',
       password: 'Password',
       submit: 'Connect',
-      stubNote:
-        'Stub mode — backend not wired. Connect signs in with a fake admin session, scoped to this instance.',
+      submitting: 'Connecting…',
+    },
+    errors: {
+      unreachable: "Can't reach this server. Check the URL and that it's online.",
+      badCredentials: 'Incorrect username or password.',
+      generic: 'Sign-in failed. Please try again.',
     },
   },
   library: {