Add visual smoke test for the solar-system scene

[jokiatgithub]

Context

The current build has unit tests, but the 3D scene needs a browser-visible safety net. A smoke test should catch blank canvas, missing labels, and basic render failures.

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

Scope

• Add a lightweight browser smoke test for the solar-system scene.
• Verify the app loads, the canvas is nonblank, and expected labels or UI controls are present.
• Keep the test practical for local contributors and future CI.

Acceptance Criteria

• The smoke test fails if the canvas is blank.
• The smoke test checks at least one expected planet/control.
• The command is documented in README or scripts docs.
• Existing lint/test/build checks still pass.