Fix memory/CPU chart fill: explicitly disable area fill for non-traffic charts

- Set fill: false for CPU, memory, and storage charts
- Prevents Chart.js from auto-filling when backgroundColor is set
- Only traffic charts should have filled area (fill: 'origin')
- Fixes visual bug where 20% memory usage appeared as 100% fill

assets/js/app.ts

@@ -97,6 +97,8 @@ const SensorChart: SensorChartHook = {
       // Add fill for traffic charts to create area effect
       if (isTrafficChart) {
         baseConfig.fill = 'origin'  // Fill to the zero line
+      } else {
+        baseConfig.fill = false  // No fill for CPU/memory/storage charts
       }
 
       return baseConfig