Create engine time module with Julian date helpers

[jokiatgithub]

Context

Starwatcher needs a dedicated time model before the solar-system engine grows beyond the current prototype. This is the first foundation stone from the near-term roadmap.

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

Scope

• Add a new engine time module, likely under src/engine/time.
• Convert UTC date/time inputs into Julian dates.
• Keep helpers pure and independent from React or Three.js.
• Document supported assumptions around UTC and valid ranges.

Acceptance Criteria

• A tested helper converts ISO date/time input to Julian date.
• Tests cover J2000, a current-year date, and at least one historical/future date.
• Existing app behavior remains unchanged.
• npm run lint, npm run test, and npm run build pass.