Enhanced sky atmosphere with DBC-driven colors, sun lighting, and zone weather

- Skybox now uses DBC sky colors (skyTop/skyMiddle/skyBand1/skyBand2) instead
  of hardcoded C++ color curves, with 3-band gradient and Rayleigh/Mie scattering
- Clouds receive sun direction for lit edges, self-shadowing, and silver lining
- Fixed sun quad box artifact with proper edge fade in celestial shader
- Lens flare attenuated by fog, cloud density, and weather intensity
- Replaced garish green/purple lens flare ghosts with warm natural palette
- Added zone-based weather system for single-player mode with per-zone rain/snow
  configuration, probability-based activation, and smooth intensity transitions
- Server SMSG_WEATHER remains authoritative when connected to a server

include/rendering/lighting_manager.hpp

@@ -28,7 +28,7 @@ struct LightingParams {
     glm::vec3 skyBand1Color{0.9f, 0.95f, 1.0f};    // Sky band 1
     glm::vec3 skyBand2Color{1.0f, 0.98f, 0.9f};    // Sky band 2
 
-    float cloudDensity = 1.0f;                      // Cloud density/opacity
+    float cloudDensity = 0.3f;                      // Cloud density/opacity
     float horizonGlow = 0.3f;                       // Horizon glow intensity
 };