Feat: Wire frontend to backend, add wallet API endpoints

- Replace frontend mock with real fetch calls to POST /api/wallet/analyze and GET /api/wallet/{id}/utxos
- Add Vite dev proxy for /api to avoid CORS in development
- Implement WalletResource.java with the two endpoints
- Add WalletMockData.java with the 5-UTXO dataset
- Configure CORS and port in application.properties
- Add backend/requests/wallet.http with kulala tests (29 assertions, all passing)

frontend/vite.config.js

@@ -3,4 +3,9 @@ import react from '@vitejs/plugin-react'
 
 export default defineConfig({
   plugins: [react()],
+  server: {
+    proxy: {
+      '/api': 'http://localhost:8080'
+    }
+  }
 })