feat(flutter): discover screen — search Lidarr + create requests

- models/lidarr.dart (LidarrSearchResult + LidarrRequestKind)
- api/endpoints/discover.dart (search + createRequest)
- discover/discover_screen.dart with TextField, kind toggle (Artists/Albums),
  results list with in_library / requested pills
- /discover route + explore icon on home AppBar (5 actions now: Library,
  Playlists, Search, Discover, Settings)

flutter_client/lib/shared/routing.dart

@@ -7,6 +7,7 @@ import '../auth/login_screen.dart';
 import '../auth/server_url_screen.dart';
 import '../library/album_detail_screen.dart';
 import '../library/artist_detail_screen.dart';
+import '../discover/discover_screen.dart';
 import '../library/home_screen.dart';
 import '../library/library_screen.dart';
 import '../player/now_playing_screen.dart';
@@ -60,6 +61,7 @@ GoRouter buildRouter(Ref ref) {
           GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
           GoRoute(path: '/library', builder: (_, __) => const LibraryScreen()),
+          GoRoute(path: '/discover', builder: (_, __) => const DiscoverScreen()),
           GoRoute(path: '/settings', builder: (_, __) => const SettingsScreen()),
           GoRoute(path: '/playlists', builder: (_, __) => const PlaylistsListScreen()),
           GoRoute(