feat: add participant data persistence display and mobile responsive fixes

- Add getStats() method to participantService for tag distribution
- Update participants API to return tagDistribution statistics
- Load existing participants on AdminControl mount
- Add mobile responsive styles for import section

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

packages/server/src/routes/admin.routes.ts

@@ -137,15 +137,17 @@ router.post('/participants/import', upload.single('file'), async (req, res, next
 
 /**
  * GET /api/admin/participants
- * Get all participants
+ * Get all participants with statistics
  */
 router.get('/participants', async (_req, res, next) => {
   try {
     const participants = participantService.getAll();
+    const stats = participantService.getStats();
     return res.json({
       success: true,
       data: {
         count: participants.length,
+        tagDistribution: stats.tagDistribution,
         participants,
       },
     });