fix: adapt code to modernized libraries

- Migrate validation.ts from class-transformer/class-validator to Zod
- Apply Biome auto-fixes (import sorting, nodejs protocol)
- Fix duplicate type identifier in validation.ts

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/validation.test.ts

@@ -0,0 +1,30 @@
+import { expect, test } from "vitest";
+import { UserStateUpdate, validateUserStateUpdate } from "../src/validation";
+
+test("valid object returns typed object", async () => {
+  const input = {
+    userId: "123",
+    username: "testuser",
+    avatar: "avatar.png",
+    speaking: true,
+  };
+  const result = await validateUserStateUpdate(input);
+  expect(result).toEqual(input as UserStateUpdate);
+});
+
+test("non-object input returns null", async () => {
+  // @ts-expect-error testing invalid input
+  const result = await validateUserStateUpdate("not an object");
+  expect(result).toBeNull();
+});
+
+test("invalid field types return null", async () => {
+  const input = {
+    userId: "123",
+    username: "testuser",
+    avatar: "avatar.png",
+    speaking: "true", // invalid type
+  };
+  const result = await validateUserStateUpdate(input as unknown);
+  expect(result).toBeNull();
+});