refactor(metrics-modal): mark min/max on chart + improve grid contrast

Drop the Current/Min/Avg/Max stats row and Live auto-refresh toggle —
clutter that didn't earn its space. Min/max are now rendered as colored
dots on the chart itself (green ▼ for min, orange ▲ for max), which
exposes both the value AND the time-axis position of each extremum at a
glance. Tooltip now formats the timestamp fully (with date prefix when
the sample crosses a day boundary).

Switch CartesianGrid stroke from var(--ant-color-border-secondary) to
rgba(128,128,140,0.35) so the gridlines stay readable in light theme
against the chart-wrap's faint primary tint — the AntD variable
resolved to near-zero alpha and the gridlines disappeared.

XrayMetricsModal keeps its implicit 2s observatory polling.

frontend/src/pages/index/XrayMetricsModal.css

@@ -63,7 +63,3 @@
   .obs-dot.is-alive { animation: none; }
 }
 
-.listen-tag {
-  opacity: 0.7;
-  font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
-}