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>

backgrounds/custom/README.md

@@ -0,0 +1,35 @@
+# Custom Backgrounds
+
+Place your custom background images in this folder.
+
+## Required Files
+
+You need to provide 4 directional background images:
+- `north.png` - View looking north
+- `east.png` - View looking east
+- `south.png` - View looking south
+- `west.png` - View looking west
+
+## Image Specifications
+
+- **Dimensions:** 1536 x 1024 pixels
+- **Format:** PNG with transparency support
+- **Orientation:** Each image should show the horizon/landscape as seen when facing that cardinal direction from your location
+
+## Tips
+
+- Include a horizon line in each image - the sun and moon will set behind it
+- Keep the upper portion (sky area) relatively simple for aircraft visibility
+- The bottom portion can have more detail (terrain, buildings, etc.)
+- Consider your local landmarks and terrain for each direction
+
+## Enabling Custom Theme
+
+Set the theme in `config.json`:
+```json
+{
+  "theme": "custom"
+}
+```
+
+Then restart the server.