fix: use background-paper for drawer to match Headlamp standard

Changed drawer backgroundColor from var(--mui-palette-background-default)
to var(--mui-palette-background-paper) to match Headlamp's standard drawer
and dialog style with proper opaque elevated surface.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src/components/NamespacesListView.tsx

@@ -103,7 +103,7 @@ function NamespaceDetailPanel({ namespace, onClose }: NamespaceDetailPanelProps)
         top: 0,
         bottom: 0,
         width: '1000px',
-        backgroundColor: 'var(--mui-palette-background-default)',
+        backgroundColor: 'var(--mui-palette-background-paper)',
         color: 'var(--mui-palette-text-primary)',
         boxShadow: '-2px 0 8px rgba(0,0,0,0.15)',
         overflowY: 'auto',