feat(flutter): refresh kebab on system playlist cards (#416 parity)

Closes the Flutter half of Fable #416. Web got a generalized
system-playlist refresh kebab in d12afda; this brings Flutter to
parity instead of leaving the affordance web-only.

- PlaylistsApi.refreshSystem(variant): POST
  /api/playlists/system/{for-you|discover}/refresh, maps the
  underscore model variant to the hyphenated route segment,
  returns the rotated playlist id.
- PlaylistCard: top-right PopupMenuButton on system playlists
  with a context-labelled "Refresh For You" / "Refresh Discover"
  item. Calls refreshSystem, invalidates playlistsListProvider
  (which reconciles the rotated UUID + new tracks), snackbars
  the result. ScaffoldMessenger captured pre-await.
- Tests: kebab present for system, absent for user playlists.

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

flutter_client/test/playlists/widgets/playlist_card_test.dart

@@ -55,4 +55,24 @@ void main() {
     expect(find.text('For You'), findsOneWidget);
     expect(find.text('for you'), findsOneWidget);
   });
+
+  testWidgets('shows refresh kebab on system playlists', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _forYou),
+      ),
+    ));
+    expect(find.byIcon(Icons.more_vert), findsOneWidget);
+  });
+
+  testWidgets('no refresh kebab on user playlists', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      theme: buildThemeData(),
+      home: const Scaffold(
+        body: PlaylistCard(playlist: _userPlaylist),
+      ),
+    ));
+    expect(find.byIcon(Icons.more_vert), findsNothing);
+  });
 }