feat: add volunteer map tab showing available/active volunteers by postcode

Summary

The volunteer search page already has ViewMode.MAP defined in src/components/Dashboard/common/types.ts and the VolunteerListController already switches on it. Wire up the map view.

Behaviour

• Show only volunteers with statusEngagement = available OR statusMatch = active (same filters already available in the filter panel)
• Place one pin per volunteer at their postcode lat/lon (from https://github.com/need4deed-org/be/issues/661)
• Skip volunteers with no coordinates
• Clicking a pin navigates to the volunteer detail page

Tasks

• Add a 3rd tab "Karte" to Volunteers.tsx (currently has 2 tabs)
• In VolunteerListController, render MapView when ViewMode.MAP
• Pass volunteer markers from the list query result
• Pre-apply engagement/match status filter when map tab is active

Depends on

• https://github.com/need4deed-org/fe/issues/621 (shared MapView)
• https://github.com/need4deed-org/be/issues/661 (coordinates in volunteer list response)