Redesign system power chart as stacked per-PSU area chart

- Add PSUReading struct and PSUs []PSUReading to LiveMetricSample
- Sample per-PSU input watts from IPMI SDR entity 10.x (Power Supply)
- Render stacked filled-area SVG chart (one layer per PSU, cumulative total)
- Fall back to single-line chart on systems with ≤1 PSU in SDR

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

audit/internal/webui/server_test.go

@@ -120,7 +120,7 @@ func TestChartDataFromSamplesUsesFullHistory(t *testing.T) {
 		},
 	}
 
-	datasets, names, labels, title, _, _, ok := chartDataFromSamples("gpu-all-power", samples)
+	datasets, names, labels, title, _, _, _, ok := chartDataFromSamples("gpu-all-power", samples)
 	if !ok {
 		t.Fatal("chartDataFromSamples returned ok=false")
 	}
@@ -164,7 +164,7 @@ func TestChartDataFromSamplesKeepsStableGPUSeriesOrder(t *testing.T) {
 		},
 	}
 
-	datasets, names, _, title, _, _, ok := chartDataFromSamples("gpu-all-power", samples)
+	datasets, names, _, title, _, _, _, ok := chartDataFromSamples("gpu-all-power", samples)
 	if !ok {
 		t.Fatal("chartDataFromSamples returned ok=false")
 	}
@@ -209,7 +209,7 @@ func TestChartDataFromSamplesIncludesGPUClockCharts(t *testing.T) {
 		},
 	}
 
-	datasets, names, _, title, _, _, ok := chartDataFromSamples("gpu-all-clock", samples)
+	datasets, names, _, title, _, _, _, ok := chartDataFromSamples("gpu-all-clock", samples)
 	if !ok {
 		t.Fatal("gpu-all-clock returned ok=false")
 	}