Fix wrong z-index after toggling path visibility

[usero-agent]

Summary

• Fixed z-index ordering breaking after toggling path visibility with the h key
• Changed the visibility toggle mechanism from removing/re-adding layers to using OpenLayers' layer.setVisible() API
• Added a setPathLayersVisible() helper function and separated the visibility toggle into its own useEffect

Root Cause

When pressing and releasing the h key, the path layers (unselected paths, selected path, access network) were being removed from the map and then re-added via map.addLayer(). In OpenLayers, re-adding layers places them at the end of the layer collection, which can break z-index ordering. This caused the selected path layer (zIndex 2, blue baseline) to render on top of the path detail colored segments layer (zIndex 3), making the colored segments invisible beneath the blue path.

Fix

Instead of removing and re-adding layers to toggle visibility, the fix uses layer.setVisible(false/true). This keeps the layers in the map's layer collection, preserving their z-index ordering. The layer creation/update logic (which handles data changes like path selection) remains separate from the visibility toggle logic.

Test plan

• TypeScript compilation passes (npx tsc --noEmit)
• All 66 existing tests pass (npm test)
• Manual: Select a route with path details visible (colored segments on the path)
• Manual: Press and release h to toggle path visibility
• Manual: Verify colored segments remain visible on top of the blue baseline after toggling

Fixes #4

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