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/library/home_screen.dart

@@ -44,6 +44,11 @@ class HomeScreen extends ConsumerWidget {
             tooltip: 'Search',
             onPressed: () => context.push('/search'),
           ),
+          IconButton(
+            icon: Icon(Icons.explore, color: fs.parchment),
+            tooltip: 'Discover',
+            onPressed: () => context.push('/discover'),
+          ),
           IconButton(
             icon: Icon(Icons.settings, color: fs.parchment),
             tooltip: 'Settings',