[Good First Issue] Improve accessibility in alert components

## Description

Alert components need accessibility improvements (ARIA labels, roles) for WCAG 2.1.

## Tasks
- [ ] Add aria-label attributes to icons in AlertPopup.tsx
- [ ] Improve color contrast for yellow/orange severity
- [ ] Validate keyboard navigation

## Difficulty: ⭐ Very Easy
## Time: 1-2 hours

Files: apps/web/src/components/map/AlertPopup.tsx, AlertDetailMap.tsx