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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.12.2",
+  "version": "1.13.0",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {