telemetry: use station callsign as the PostHog distinct_id (dedupe users)

The random per-install UUID lives in the LOCAL SQLite, so one operator running
OpsLog on several machines (laptop / desktop / Maestro) or after a reinstall
counted as several distinct "users" — making the user total unreliable. Use the
station callsign as distinct_id when set (public in ham radio, stable across all
of an op's machines); the install UUID stays as a fallback and rides along as an
install_id property so per-machine detail isn't lost.

telemetry.go

@@ -85,14 +85,26 @@ func (a *App) sendTelemetryHeartbeat() {
 		return // already counted today
 	}
 
+	// distinct_id identifies the USER. Prefer the station callsign — it's stable
+	// across every machine/reinstall the same operator runs, so one op counts as
+	// one user (a callsign is public in amateur radio). The random per-install ID
+	// is only a fallback until a callsign is configured; without it the same op on
+	// a laptop + desktop + Maestro showed up as three "users". install_id rides
+	// along as a property so multiple machines under one callsign stay visible.
+	installID := a.telemetryInstallID()
+	distinctID := installID
+	if call := a.activeCallsign(); call != "" {
+		distinctID = call
+	}
 	payload := map[string]any{
 		"api_key":     posthogAPIKey,
 		"event":       "app_opened",
-		"distinct_id": a.telemetryInstallID(),
+		"distinct_id": distinctID,
 		"properties": map[string]any{
 			"version":    appVersion,
 			"os":         runtime.GOOS,
 			"arch":       runtime.GOARCH,
+			"install_id": installID,
 			"$lib":       "opslog",
 		},
 		"timestamp": time.Now().UTC().Format(time.RFC3339),