[Feature] Procedural sky effects to improve theme atmosphere

[jashkarangiya]

What problem does this solve?

The current theme skies (Midnight, Sunset, Neon, Emerald) look good color-wise but feel static/empty, which reduces immersion.

Proposed solution

Add lightweight, procedural sky effects per theme, generated at runtime (no external assets).

• Midnight: twinkling starfield, animated aurora bands, moon, occasional shooting stars
• Sunset: volumetric sun disc + halo, seamless 360-degree cirrus cloud dome
• Neon/Emerald: theme-matched procedural sky treatments

Implementation approach:

• New component: ThemeSkyFX.tsx to keep CityCanvas.tsx clean
• All textures generated via CanvasTexture at runtime
• Dispose THREE.Texture / THREE.Material in useEffect cleanup to avoid leaks

Alternatives considered

• Static sky textures (adds assets and bundle size)
• Minor animation on existing sprites (limited improvement)

Additional context

I’ve opened a PR implementing this feature. Please take a look when you have a moment.