feat(wallets): add isDefault field to wallet model and implement setDefaultWallet functionality

apps/backend/prisma/migrations/20260617131039_add_wallet_is_default/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "wallets" ADD COLUMN     "isDefault" BOOLEAN NOT NULL DEFAULT false;

apps/backend/prisma/schema.prisma

@@ -134,6 +134,7 @@ model Wallet {
   name      String    @db.VarChar(100)
   currency  String    @db.VarChar(10)
   balance   Decimal   @db.Decimal(14, 4)
+  isDefault Boolean   @default(false)
   createdAt DateTime  @default(now())
   updatedAt DateTime  @updatedAt