Add per-metric forecast burndown chart to CCP Metrics page

New feature: combined historical + forecast burndown chart with metric
selector on the CCP Metrics page. Shows stacked bars (total assets vs
non-compliant) with a compliance percentage trend line. A bold divider
separates actual historical data from projected future remediation.
Forecast assumes constant asset count and on-schedule remediation plans.

Backend:
- computeMetricForecastBurndown helper in vclHelpers.js (pure function)
- GET /api/compliance/vcl-multi/metrics-list endpoint
- GET /api/compliance/vcl-multi/metric/:metricId/forecast-burndown endpoint

Frontend:
- MetricSelector dropdown with device counts per metric
- ForecastBurndownChart using recharts ComposedChart (Bar + Line + ReferenceLine)
- Forecast bars render at 50% opacity to distinguish from actuals
- Race condition handling for rapid metric switching
- Queue panel width increased from 420px to 600px

Closes #18

frontend/src/components/pages/ReportingPage.js

@@ -2197,7 +2197,7 @@ function QueuePanel({ open, items, onClose, onUpdate, onDelete, onDeleteMany, on
             <div
                 style={{
                     position: 'fixed', top: 0, right: 0,
-                    height: '100vh', width: '420px',
+                    height: '100vh', width: '600px',
                     zIndex: 9999,
                     display: 'flex', flexDirection: 'column',
                     background: 'linear-gradient(180deg, #0D1929 0%, #080F1C 100%)',