Flag LiveTrack sessions whose map Garmin no longer serves

Ended sessions past the ~24h Garmin appears to keep a live-track page
around for get verified once at app startup and, once confirmed dead,
flagged so the panel greys them out instead of only failing after a
click. Already-flagged sessions are skipped on later startups so this
doesn't keep re-hitting Garmin for the same dead session.

public/css/style.css

@@ -781,6 +781,11 @@ html, body {
   cursor: wait;
 }
 
+.livetrack-panel__row:disabled {
+  opacity: 0.5;
+  cursor: default;
+}
+
 .livetrack-panel__label {
   flex: 1;
   min-width: 0;