Add descriptive alt text to map markers for screen readers

[vgpastor]

Context

The map markers on the AED map currently lack descriptive alt text and ARIA labels, making them inaccessible to screen reader users.

What needs to happen

• Add meaningful alt attributes to Leaflet markers (e.g., "AED defibrillator at Calle Gran Vía 42, Madrid")
• Add aria-label to marker popups
• Test with a screen reader (VoiceOver or NVDA)

Where to look

• src/components/ — Map-related components
• Leaflet marker configuration

Tech

• React, Leaflet, react-leaflet
• No backend changes needed

Acceptance criteria

• Markers have descriptive alt text including location
• Popups have ARIA labels
• Keyboard navigation works for map markers