fix: correct yellowish halo to blue on 3D/2D visualisation

[diogobaltazar]

Problem

The glow/halo around the 3D/2D visualisation node overlay is rendering in a warm yellow (#ffe58a45 drop-shadow) instead of the project's blue palette.

Solution

• Replace the yellowish drop-shadow on .focus-node-overlay .node-core.focus-node (App.css ~line 1066) with a blue-toned equivalent.
• Ensure all visualisation glows and halos stay within the established blue shade range.
• Design rule has been documented in .claude/agents/ui-engineer.md under Theme → Colours.

Acceptance criteria

• No warm/yellow tones remain in any visualisation halo or glow.
• The halo colour is visually consistent with the rest of the blue UI palette.
• The design decision is recorded in ui-engineer.md.