feat(race-day): start heats with an EFRA/IFMAR count-down and flying start

Race Day Start now runs a count-down (Go, 30 s - 5 min, default per
event) with calls every minute, 30 s and 10 s, grid call and 9..3
count for grid starts, hold/resume/+1 min/to 30 s/to 10 s/Go now, and
starts the heat by itself when it runs out. The transponder check runs
during the count-down, replacing the separate warm-up.

Adds a flying start mode (horn + clock is running, own clock per car)
as the qualifying default, and the latest personal clock start rules
from EFRA 9.3 / IFMAR 4.2.3 for flying and staggered starts.

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

src/styles.css

@@ -4448,10 +4448,24 @@ select:focus {
   color: var(--muted, inherit);
 }
 
-.warmup-clock {
-  font-size: 1.6rem;
+.prologue-clock {
+  font: 700 1.6rem/1 Orbitron, sans-serif;
   font-variant-numeric: tabular-nums;
-  margin-right: 8px;
+  align-self: center;
+  margin-right: 4px;
+}
+
+.prologue-clock.is-held,
+.race-day-clock.is-prologue strong {
+  color: #f5a623;
+}
+
+.prologue-length {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  font-size: 0.75rem;
+  color: var(--muted);
 }
 
 .warmup-table {