feat(dashboard): richer System History & Xray Metrics charts

- Collect disk read/write and network packet-rate metrics on the host sampler
- Sparkline: optional 2nd/3rd overlaid series with a colored legend
- System History: merge Bandwidth (up/down), Disk I/O (read/write) and Load (1m/5m/15m) into single multi-line tabs
- Add a descriptive per-chart title and mobile-only tab icons to both modals
- Localize every chart title and tab label across all 13 languages

frontend/src/components/viz/Sparkline.css

@@ -32,3 +32,28 @@
   gap: 4px;
   white-space: nowrap;
 }
+
+.sparkline-legend {
+  position: absolute;
+  top: 2px;
+  right: 8px;
+  display: inline-flex;
+  align-items: center;
+  gap: 12px;
+  padding: 2px 8px;
+  background: color-mix(in srgb, var(--ant-color-bg-elevated) 88%, transparent);
+  border: 1px solid var(--ant-color-border-secondary);
+  border-radius: 999px;
+  font-size: 11px;
+  font-weight: 600;
+  line-height: 16px;
+  pointer-events: none;
+  z-index: 1;
+}
+
+.sparkline-legend .extrema-item {
+  display: inline-flex;
+  align-items: center;
+  gap: 4px;
+  white-space: nowrap;
+}