Serialize date, mode, camera, and selected body into URLs

[jokiatgithub]

Context

Shareable scenes are central to Starwatcher as a public project. A URL should be able to reopen the same moment and viewpoint.

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

Scope

• Serialize date, time, render mode, selected body, and basic camera state into the URL.
• Restore state from URL parameters on load.
• Keep URLs readable enough for users to share.
• Handle invalid or old parameters gracefully.

Acceptance Criteria

• Reloading a URL restores the same date/time and render mode.
• Selected body restores when present.
• Invalid params fall back safely.
• No secrets or local-only data are stored in URLs.