Add log4js fileSync logger module and formatError helper

src/server/log-format.test.ts

@@ -0,0 +1,22 @@
+import { describe, it, expect } from "vitest";
+import { formatError } from "./log-format";
+
+describe("formatError", () => {
+  it("returns stack for Error", () => {
+    const e = new Error("boom");
+    expect(formatError(e)).toBe(e.stack);
+  });
+  it("falls back to message when stack missing", () => {
+    const e = new Error("boom");
+    e.stack = undefined as unknown as string;
+    expect(formatError(e)).toBe("boom");
+  });
+  it("stringifies non-Error primitives", () => {
+    expect(formatError("oops")).toBe("oops");
+    expect(formatError(42)).toBe("42");
+  });
+  it("handles null / undefined", () => {
+    expect(formatError(null)).toBe("Unknown error");
+    expect(formatError(undefined)).toBe("Unknown error");
+  });
+});