#24 — Layer control: per-type visibility toggles

[bobinush]

Acceptance criteria:

• A floating panel on the map lists every distinct entity type that has at least one entity on the map
• Each row has a checkbox, a colour dot (from the style's fill colour) and the style's display name
• Unchecking a type hides all entities of that type; checking restores them
• New entity types appear in the list automatically when the first entity of that type is added