(Re)introduction Hotfix

A quick hotfix today, fixing some issues reported by our Discord:

• Fixed missing UI shader.
  
• Warning that no resources were returned will not appear unless you have researched a resource gathering technology.
  
• Underground base entrances now have proper pathfinding around them.
  
• Fixed typo in Ground Command Construction research topic says.
  
• Increased resolution of unit and structure thumbnail images to 1024x1024.
  

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