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.md

@@ -15,7 +15,8 @@ Edit `config.json` to customize your installation:
     "lat": 0.0,
     "lon": 0.0
   },
-  "web_port": 2001
+  "web_port": 2001,
+  "theme": "desert"
 }
 ```
 
@@ -31,6 +32,7 @@ Edit `config.json` to customize your installation:
 | `location.lat` | number | Latitude of your receiver location |
 | `location.lon` | number | Longitude of your receiver location |
 | `web_port` | number | Port for the web interface (default: 2001) |
+| `theme` | string | Background theme: `"desert"` (default) or `"custom"` |
 
 ### Receiver Configuration Examples
 
@@ -51,12 +53,44 @@ Edit `config.json` to customize your installation:
 
 ---
 
-## Background Images
+## Background Themes
 
-Pixel View uses directional background images to show the horizon view from your receiver location. You should customize these to match your actual surroundings.
+Pixel View uses directional background images to show the horizon view from your receiver location. Backgrounds are organized into themes stored in the `backgrounds/` folder.
+
+### Available Themes
+
+| Theme | Description |
+|-------|-------------|
+| `desert` | Las Vegas desert landscape (default) |
+| `custom` | Your own custom backgrounds |
+
+### Theme Folder Structure
+
+```
+backgrounds/
+├── desert/          # Default desert theme
+│   ├── north.png
+│   ├── east.png
+│   ├── south.png
+│   └── west.png
+└── custom/          # Your custom backgrounds
+    ├── README.md    # Instructions for creating custom backgrounds
+    ├── north.png
+    ├── east.png
+    ├── south.png
+    └── west.png
+```
+
+### Using Custom Backgrounds
+
+1. Add your background images to `backgrounds/custom/`
+2. Set `"theme": "custom"` in your `config.json`
+3. Restart the server
 
 ### Image Files
 
+Each theme folder needs these 4 directional images:
+
 | File | Direction | Description |
 |------|-----------|-------------|
 | `north.png` | North (0°) | View looking north from your location |