refactor: extract isNoExpiry utility for zero-time handling

Go's time.Time{} serialises to '0001-01-01T00:00:00Z' which headscale
may return instead of null for tagged nodes (juanfont/headscale#3170).

Commit extracts the the existing inline check into a shared function for reuse.

No behavior change — mapNodes() already handled both variants.

tests/unit/utils/node-info.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, test } from "vitest";
+
+import { isNoExpiry } from "~/utils/node-info";
+
+describe("isNoExpiry", () => {
+  test("returns true for null", () => {
+    expect(isNoExpiry(null)).toBe(true);
+  });
+
+  test("returns true for undefined", () => {
+    expect(isNoExpiry(undefined)).toBe(true);
+  });
+
+  test("returns true for Go zero-time ISO format", () => {
+    expect(isNoExpiry("0001-01-01T00:00:00Z")).toBe(true);
+  });
+
+  test("returns true for Go zero-time space format", () => {
+    expect(isNoExpiry("0001-01-01 00:00:00")).toBe(true);
+  });
+
+  test("returns false for a real future expiry", () => {
+    expect(isNoExpiry("2030-01-01T00:00:00Z")).toBe(false);
+  });
+
+  test("returns false for a real past expiry", () => {
+    expect(isNoExpiry("2020-01-01T00:00:00Z")).toBe(false);
+  });
+});