feat: implement credentials login

implements the credentials login functionality

src/register.ts

@@ -29,6 +29,18 @@ export async function registerAction(data: z.infer<typeof registerSchema>) {
       };
     }
 
+    const existingUsername = await prisma.user.findUnique({
+      where: {
+        name: username,
+      },
+    });
+
+    if (existingUsername) {
+      return {
+        error: 'Username already exists',
+      };
+    }
+
     const passwordHash = await bcrypt.hash(password, 10);
 
     await prisma.$transaction(async (tx) => {