feat(race-day): add warm-up transponder check before a heat

Race Day gets a 1-3 minute warm-up per heat with a green check per
driver when their transponder is seen, countdown announcements
(sv/en), quick transponder fix from the panel, and a start warning
that still allows starting when someone is missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/run.js

@@ -1,4 +1,5 @@
 import "./auth.test.js";
+import "./warmup_logic.test.js";
 import "./tts.test.js";
 import "./piper_speech.test.js";
 import "./driver_profile_logic.test.js";