style(mobile-app): reduce ranking list item spacing from 12px to 4px

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/features/ranking/presentation/pages/ranking_page.dart

@@ -307,7 +307,7 @@ class _RankingPageState extends ConsumerState<RankingPage> {
       itemCount: _mockRankingData.length,
       itemBuilder: (context, index) {
         return Padding(
-          padding: const EdgeInsets.only(bottom: 12),
+          padding: const EdgeInsets.only(bottom: 4),
           child: _buildRankingItem(_mockRankingData[index]),
         );
       },