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/sky_system.hpp

@@ -31,9 +31,10 @@ struct SkyParams {
     glm::vec3 skyBand2Color{1.0f, 0.98f, 0.9f};
 
     // Atmospheric effects
-    float cloudDensity = 0.0f;  // 0-1
-    float fogDensity = 0.0f;    // 0-1
-    float horizonGlow = 0.3f;   // 0-1
+    float cloudDensity = 0.0f;      // 0-1
+    float fogDensity = 0.0f;        // 0-1
+    float horizonGlow = 0.3f;       // 0-1
+    float weatherIntensity = 0.0f;  // 0-1 (rain/snow intensity, attenuates lens flare)
 
     // Time
     float timeOfDay = 12.0f;    // 0-24 hours