Add camera focus mode for selected planets

[jokiatgithub]

Context

Once body selection exists, the next natural interaction is camera focus. This helps the solar-system scene become explorable instead of just decorative.

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

Scope

• Add a focus action for the selected body.
• Smoothly move or retarget the camera to that body.
• Preserve a way back to heliocentric overview.
• Keep camera state stable enough for future URL serialization.

Acceptance Criteria

• A selected planet can become the camera focus.
• Users can return to overview mode.
• Camera movement does not clip through or lose the scene.
• Existing controls remain usable after focus changes.