fix: chart Y-axis auto-scaling, error rate unit, memory reference line, pointer events

- Throughput chart: divide totalCount by bucket duration (seconds) so Y-axis shows true msg/s instead of raw bucket counts; fixes flat-line appearance when TPS is low but totalCount is large
- Error Rate chart: convert failedCount/totalCount to percentage; change yLabel from "err/h" to "%" to match KPI stat card unit
- Memory chart: add threshold line at jvm.memory.heap.max so chart Y-axis extends to max heap and shows the reference line (spec 5.3)
- Agent state: suppress containerStatus badge when value is "UNKNOWN"; only render it with "Container: <state>" label when a non-UNKNOWN secondary state is present (spec 5.4)
- DashboardTab chartGrid: add pointer-events:none with pointer-events:auto on children so the chart grid overlay does not intercept clicks on the Application Health table rows below (spec 5.5)

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

ui/src/pages/DashboardTab/DashboardTab.module.css

@@ -14,6 +14,11 @@
   display: grid;
   grid-template-columns: 1fr 1fr;
   gap: 16px;
+  pointer-events: none;
+}
+
+.chartGrid > * {
+  pointer-events: auto;
 }
 
 .chartRow {