Nebula and starfield overhaul
Reworked the nebula and starfield over a run of tweaks: volumetric per-system clouds, a star river, colour, depth, and seam fixes.
- Reworked nebulas as volumetric clouds with real depth, each system its own server-decided colour.
- Added a milky-way star river and brighter colour stars that drift on their own depth layer.
- Added two-tone nebulas that blend a second matched colour through the clouds.
- Enlarged the nebulas and widened the milky-way band.
- Fixed background stars rendering as harsh vertical streaks.
- Fixed faint seams across the gas and the dark gaps between clouds.
- Thinned overcrowded starfields so sparse systems feel emptier.