#18 — POIs tab: create and edit POI

[bobinush]

Acceptance criteria:

• Form fields: Name (required), Description (textarea), Link (optional URL), Icon (selected via picker — see Update the map with this years (2024) layout #19)
• "Place Marker" activates Geoman marker mode; clicking the map places the POI and shows a green status indicator
• For existing POIs, "Move Marker" replaces the current position
• POST /api/entities on create; PUT /api/entities/:id on update
• The marker on the map uses the selected PNG icon (/icons/{name}.png)
• Clicking the marker on the map (in any tab) shows a popup with the name, description, and a clickable link if one is set