Warn when a driver's transponder collides with a different driver

Registering or editing a driver with a transponder already assigned
to a different driver (e.g. a wrong-class typo) previously saved
silently - the only symptom was that driver's laps never registering
during their heat, with no indication why. Now a confirm dialog names
the conflicting driver and their class before saving; the same
person's own multiple class entries sharing one physical transponder
are correctly never flagged.

Also confirms two things the user asked to verify: the race clock
already counts down for all timed session types (only open practice
counts up unbounded), and a driver not entered in a heat already has
their laps fully discarded rather than recorded - both were already
correct, no changes needed there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/driver_profile_logic.test.js

@@ -0,0 +1,38 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { findTransponderConflict } from "../src/driver_profile_logic.js";
+
+// Regression coverage for a real scenario the user described: a driver registered with the
+// wrong transponder number by mistake - one that's actually assigned to a completely
+// different person (a driver in another class). Before this, nothing warned about it; the
+// only symptom was that driver's laps silently never registering during their heat.
+test("findTransponderConflict flags a transponder already used by a driver from a different profile", () => {
+  const drivers = [
+    { id: "d1", name: "Erik Andersson", transponder: "1000001", profileId: "p1" },
+    { id: "d2", name: "Marcus Dahl", transponder: "1000009", profileId: "p2" },
+  ];
+  const conflict = findTransponderConflict(drivers, { transponder: "1000009", profileId: "p1" });
+  assert.equal(conflict?.id, "d2");
+});
+
+test("findTransponderConflict does not flag the same person's own multi-class entries (same profile)", () => {
+  const drivers = [
+    { id: "d1", name: "Erik Andersson", transponder: "1000001", profileId: "p1" },
+    { id: "d2", name: "Erik Andersson", transponder: "1000001", profileId: "p1" },
+  ];
+  const conflict = findTransponderConflict(drivers, { transponder: "1000001", profileId: "p1" });
+  assert.equal(conflict, null);
+});
+
+test("findTransponderConflict excludes the driver being edited from its own conflict check", () => {
+  const drivers = [
+    { id: "d1", name: "Erik Andersson", transponder: "1000001", profileId: "p1" },
+  ];
+  const conflict = findTransponderConflict(drivers, { transponder: "1000001", profileId: "p1", excludeDriverId: "d1" });
+  assert.equal(conflict, null);
+});
+
+test("findTransponderConflict is a no-op for an empty transponder", () => {
+  const drivers = [{ id: "d1", name: "Erik Andersson", transponder: "", profileId: "p1" }];
+  assert.equal(findTransponderConflict(drivers, { transponder: "", profileId: "p2" }), null);
+});