Add aria-label to icon-only buttons

[isaacsight]

Several icon-only close buttons lack aria-labels for screen readers. Files: src/components/KnowledgePanel.tsx, src/components/kernel-agent/StatsPanel.tsx. Add aria-label="Close" to all <button> elements containing only an icon.

[isaacsight]

Fixed in d70844b — 12 icon-only buttons across 5 components now have aria-labels.