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>

src/driver_profile_logic.js

@@ -44,3 +44,19 @@ export function getDriverProfileReferences(state, reference) {
   profile.driverIds.forEach((driverId) => references.add(driverId));
   return references;
 }
+
+// A transponder is one physical chip, so the same value legitimately repeats across a
+// single person's own multi-class driver entries (same profile) - that is not a
+// conflict. It only means something went wrong when it also shows up on a driver
+// belonging to a DIFFERENT profile, since two different people can't share one chip:
+// most often a typo at registration that quietly grabbed someone else's transponder.
+export function findTransponderConflict(drivers, { transponder, profileId, excludeDriverId } = {}) {
+  const value = String(transponder || "").trim();
+  if (!value) return null;
+  const list = Array.isArray(drivers) ? drivers : [];
+  return list.find((driver) => (
+    String(driver.id || "") !== String(excludeDriverId || "")
+    && String(driver.transponder || "").trim() === value
+    && getDriverProfileId(driver) !== String(profileId || "")
+  )) || null;
+}