feat(flutter): SystemPlaylistsStatus model + endpoint method

Adds the model + MeApi.systemPlaylistsStatus() for the home Playlists
row's placeholder-variant logic (building / failed / pending /
seed-needed). Mirrors GET /api/me/system-playlists-status which
returns the caller's most recent system_playlist_runs row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/api/endpoints/me.dart

@@ -2,6 +2,7 @@ import 'package:dio/dio.dart';
 
 import '../../models/history_event.dart';
 import '../../models/quarantine_mine.dart';
+import '../../models/system_playlists_status.dart';
 
 /// /api/me/* endpoints — caller-scoped data (history, profile, etc.).
 class MeApi {
@@ -27,4 +28,14 @@ class MeApi {
             QuarantineMineRow.fromJson((e as Map).cast<String, dynamic>()))
         .toList(growable: false);
   }
+
+  /// GET /api/me/system-playlists-status. Returns the caller's most
+  /// recent system-playlist build state. Used by the home Playlists
+  /// row to choose between real and placeholder cards.
+  Future<SystemPlaylistsStatus> systemPlaylistsStatus() async {
+    final r = await _dio.get<Map<String, dynamic>>(
+      '/api/me/system-playlists-status',
+    );
+    return SystemPlaylistsStatus.fromJson(r.data ?? const {});
+  }
 }