fix: correct ping display logic for upside-down mode

Change from showing ping for both UP and DOWN in upside-down mode
to showing ping only for the successful status:
- Normal mode: show ping when status === UP
- Upside-down mode: show ping when status === DOWN

This prevents the green up line from overlaying on top of the
red down marker in upside-down mode.

src/components/PingChart.vue

@@ -391,7 +391,7 @@ export default {
 
                 pingData.push({
                     x,
-                    y: (beat.status === UP || (beat.status === DOWN && isUpsideDown)) ? beat.ping : null,
+                    y: (isUpsideDown ? beat.status === DOWN : beat.status === UP) ? beat.ping : null,
                 });
                 downData.push({
                     x,