Add fallback for undefined ratio in TraceFilter chart config

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Discover/Map/Filter/TraceFilter.vue

@@ -309,8 +309,8 @@ function barOptions() {
           // Set tick intervals
           display: false, // Hide values, only show grid lines
           min: 0,
-          max: traceList.value[0]?.ratio,
-          stepSize: traceList.value[0]?.ratio / 4,
+          max: traceList.value[0]?.ratio ?? 1,
+          stepSize: (traceList.value[0]?.ratio ?? 1) / 4,
         },
         grid: {
           color: "rgba(100,116,139)",