chore(config): 🎨 restructure lazygit theme configuration

Move theme settings under `gui` section to match latest lazygit
configuration format. This aligns with the updated configuration
structure introduced in recent versions.

The color values remain unchanged - only the nesting structure has been
modified to maintain compatibility with lazygit's configuration schema
requirements.

home/.config/lazygit/config.yml

@@ -1,3 +1,4 @@
+gui:
   theme:
     activeBorderColor:
       - '#8bd5ca'