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>

server.py

@@ -24,7 +24,8 @@ config = {
         "lat": 0.0,
         "lon": 0.0
     },
-    "web_port": 2001
+    "web_port": 2001,
+    "theme": "desert"
 }
 
 def load_config():
@@ -45,6 +46,7 @@ def load_config():
     print(f"  Receiver port: {config['receiver_port']}")
     print(f"  Location: {config['location']['name']} ({config['location']['lat']}, {config['location']['lon']})")
     print(f"  Web port: {config['web_port']}")
+    print(f"  Theme: {config.get('theme', 'desert')}")
 
 # Flight data storage
 flights: Dict[str, dict] = {}
@@ -302,6 +304,13 @@ async def handle_receiver_location(request):
         "name": config["location"]["name"]
     })
 
+async def handle_config(request):
+    """Return client-relevant configuration"""
+    return web.json_response({
+        "theme": config.get("theme", "desert"),
+        "location": config["location"]
+    })
+
 async def handle_http(request):
     """Serve static files"""
     path = request.path
@@ -319,6 +328,7 @@ async def start_http_server():
     app = web.Application()
     app.router.add_get('/ws', websocket_handler)
     app.router.add_get('/api/receiver-location', handle_receiver_location)
+    app.router.add_get('/api/config', handle_config)
     app.router.add_get('/{tail:.*}', handle_http)
 
     runner = web.AppRunner(app)