Add theme system for customizable backgrounds

- Move backgrounds to themed folders (backgrounds/desert/, backgrounds/custom/)
- Add theme config option in config.json (default: "desert")
- Add /api/config endpoint to serve theme and location
- Update pixel-view.js to load backgrounds from theme folder
- Add config.json.example for reference
- Update CONFIG.md documentation

Users can now set "theme": "custom" and place their own backgrounds
in backgrounds/custom/ to customize the view for their location.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.json

@@ -6,5 +6,6 @@
     "lat": 0.0,
     "lon": 0.0
   },
-  "web_port": 2001
+  "web_port": 2001,
+  "theme": "desert"
 }