[dashboard] Add empty state illustration for Canary Alerts panel

[nagasatish007]

Summary

The Canary Alerts panel shows a text-only empty state when no alerts exist. Add a small SVG illustration to make it more visually engaging.

What to do

• Create a simple SVG illustration (shield with checkmark, or calm tiger, or "all clear" icon)
• Add to dashboard/web/src/components/EmptyState/ or inline in the component
• Render above the text in the Canary Alerts empty state
• Keep SVG under 2KB, single color using CSS variables

Design guidelines

• Use var(--color-text-secondary) for stroke color so it works in both themes
• Max dimensions: 80x80px
• Style: minimal line art, consistent with the dashboard's clean aesthetic
• Accessible: include aria-hidden="true" on decorative SVG

Acceptance criteria

• SVG renders in the Canary Alerts empty state
• Works in both light content area and dark sidebar context
• Doesn't break layout on narrow viewports
• Under 2KB file size

Helpful links

• Empty state component: dashboard/web/src/components/EmptyState/
• Canary panel: dashboard/web/src/panels/CanaryAlertsPanel.tsx
• Design reference: similar dashboards use simple line illustrations for empty states