feat: add auto-lookup and duplicate check for WSDC registration

Enhanced WSDC registration flow with auto-lookup and account validation:

Backend changes:
- Add accountExists flag to WSDC lookup endpoint
- Check database for existing users with WSDC ID
- Fix Prisma binary target for Alpine Linux Docker containers

Frontend changes:
- Auto-lookup WSDC data after entering 4+ digits (500ms debounce)
- Show live preview dropdown with dancer information
- Display warning if account with WSDC ID already exists
- Block registration and suggest login for existing accounts
- Improve UX with real-time validation feedback
- Add CheckCircle, XCircle, AlertCircle icons for visual feedback

This prevents duplicate WSDC ID registrations and provides immediate
feedback to users, improving the registration experience.

Tested with:
- ID 26111 (Vince Yap) - new account allowed
- ID 26997 (Radoslaw) - existing account blocked

backend/prisma/schema.prisma

@@ -2,7 +2,8 @@
 // Database: PostgreSQL 15
 
 generator client {
-  provider = "prisma-client-js"
+  provider      = "prisma-client-js"
+  binaryTargets = ["native", "linux-musl-openssl-3.0.x"]
 }
 
 datasource db {

backend/src/controllers/wsdc.js

@@ -3,6 +3,7 @@
  * Provides proxy endpoint for World Swing Dance Council (WSDC) dancer lookup
  */
 
+const { prisma } = require('../utils/db');
 const WSDC_API_BASE = 'https://points.worldsdc.com/lookup2020/find';
 
 /**
@@ -61,9 +62,15 @@ exports.lookupDancer = async (req, res) => {
       dominateRole: data.dominate_data?.short_dominate_role || null
     };
 
+    // Check if user with this WSDC ID already exists in our database
+    const existingUser = await prisma.user.findUnique({
+      where: { wsdcId: String(data.dancer_wsdcid) }
+    });
+
     return res.status(200).json({
       success: true,
-      dancer: dancerData
+      dancer: dancerData,
+      accountExists: !!existingUser
     });
 
   } catch (error) {