Add computeTransferMinutes and formatMinutesAsDuration helpers

src/features/online-board/components/TransferBar/computeTransferTime.test.ts

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+import { describe, it, expect } from "vitest";
+import { computeTransferMinutes, formatMinutesAsDuration } from "./computeTransferTime.js";
+
+describe("computeTransferMinutes", () => {
+  it("returns positive diff in minutes", () => {
+    expect(computeTransferMinutes("2026-04-17T10:00:00Z", "2026-04-17T11:30:00Z")).toBe(90);
+  });
+
+  it("returns 0 when times are identical", () => {
+    const t = "2026-04-17T10:00:00Z";
+    expect(computeTransferMinutes(t, t)).toBe(0);
+  });
+
+  it("returns null when departure is before arrival (negative)", () => {
+    expect(computeTransferMinutes("2026-04-17T11:00:00Z", "2026-04-17T10:00:00Z")).toBeNull();
+  });
+
+  it("returns null when arrival is undefined", () => {
+    expect(computeTransferMinutes(undefined, "2026-04-17T10:00:00Z")).toBeNull();
+  });
+
+  it("returns null when departure is undefined", () => {
+    expect(computeTransferMinutes("2026-04-17T10:00:00Z", undefined)).toBeNull();
+  });
+
+  it("returns null for invalid ISO strings", () => {
+    expect(computeTransferMinutes("not-a-date", "2026-04-17T10:00:00Z")).toBeNull();
+  });
+});
+
+describe("formatMinutesAsDuration", () => {
+  it("formats 90 minutes as '1h 30m'", () => {
+    expect(formatMinutesAsDuration(90)).toBe("1h 30m");
+  });
+
+  it("formats 45 minutes as '45m'", () => {
+    expect(formatMinutesAsDuration(45)).toBe("45m");
+  });
+
+  it("formats 60 minutes as '1h'", () => {
+    expect(formatMinutesAsDuration(60)).toBe("1h");
+  });
+
+  it("formats 0 as '0m'", () => {
+    expect(formatMinutesAsDuration(0)).toBe("0m");
+  });
+});

src/features/online-board/components/TransferBar/computeTransferTime.ts

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+import { parseISO, differenceInMinutes, isValid } from "date-fns";
+
+/**
+ * Diff (in minutes) between departure and arrival ISO UTC strings.
+ * Returns null when either input is missing/invalid, or when result is negative.
+ */
+export function computeTransferMinutes(
+  arrivalUtc: string | undefined,
+  departureUtc: string | undefined,
+): number | null {
+  if (!arrivalUtc || !departureUtc) return null;
+  const arr = parseISO(arrivalUtc);
+  const dep = parseISO(departureUtc);
+  if (!isValid(arr) || !isValid(dep)) return null;
+  const diff = differenceInMinutes(dep, arr);
+  if (diff < 0) return null;
+  return diff;
+}
+
+/**
+ * Format a minute count as "Nh Mm" (or "Mm" when <1 hour, or "Nh" when no remainder).
+ */
+export function formatMinutesAsDuration(minutes: number): string {
+  if (minutes < 60) return `${minutes}m`;
+  const h = Math.floor(minutes / 60);
+  const m = minutes % 60;
+  if (m === 0) return `${h}h`;
+  return `${h}h ${m}m`;
+}