fix: comprehensive code quality, theming, and test coverage improvements

- Fix ExemptionManager apiVersion bug (apps/batch resources used wrong API path)
- Replace resource: any with proper KubeResource interface (strict TypeScript)
- Replace all var(--mui-palette-*) CSS variables with useTheme() + theme.palette.*
- Replace custom drawer with MUI Drawer component (proper a11y and theming)
- Replace alert() calls with StatusLabel-based inline feedback
- Add PolarisErrorBoundary wrapping all registered plugin components
- Export getPolarisApiPath/isFullUrl from polaris.ts, deduplicate in PolarisSettings
- Fix PolarisDataContext test mock missing triggerRefresh
- Fix DashboardView test SimpleTable mock using any
- Remove dead NamespaceDetailView (replaced by drawer), unused MockPolarisProvider,
  unused getSeverityColor export
- Add tests for InlineAuditSection, AppBarScoreBadge, topIssues, checkMapping (32 new)
- Update CLAUDE.md, CHANGELOG.md, README.md for v0.6.0

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

src/api/checkMapping.ts

@@ -207,22 +207,6 @@ export function getCheckCategory(checkId: string): 'Security' | 'Efficiency' | '
   return CHECK_MAPPING[checkId]?.category || 'Security';
 }
 
-/**
- * Get color for severity
- */
-export function getSeverityColor(severity: string): string {
-  switch (severity) {
-    case 'danger':
-      return '#f44336';
-    case 'warning':
-      return '#ff9800';
-    case 'ignore':
-      return '#9e9e9e';
-    default:
-      return '#9e9e9e';
-  }
-}
-
 /**
  * Get status for StatusLabel component
  */