Add body picking and selected-body info panel

[jokiatgithub]

Context

A serious 3D engine should be inspectable. Users need to click or tap a body and understand what they are seeing.

Roadmap: https://github.com/jokiatgithub/starwatcher/blob/main/docs/solar-system-engine-roadmap.md

Scope

• Add pointer selection for rendered planets.
• Track selected body in app state.
• Show a compact panel with name, distance, orbit period if available, and data-source notes.
• Preserve keyboard and screen-reader basics where practical.

Acceptance Criteria

• Clicking a planet selects it.
• Selected state is visually visible.
• The info panel updates from body metadata.
• The feature works without breaking orbit controls.