Feat: Consolidate background chooser and 3D terrain controls into Layers component

[spydmobile]

Feature

Migrate the background/basemap chooser and 3D terrain toggle controls into the Layers component, consolidating all map layer controls into a single unified panel.

Current State

Background chooser and 3D terrain controls exist as separate UI elements.

Desired State

All map layer controls (user layers, background/basemap selection, 3D terrain toggle) live in one Layers component.