pi: GPIO-controlled theme switch

- apt dependencies (RPI.GPIO somehow needs to be installed from apt to work & re-establish uv venv with --system-site-packages
- GPIO 20 triggers mode switching (can link to a photodiode or just switch)

pi/backend/pyproject.toml

@@ -10,6 +10,8 @@ dependencies = [
     "gevent-websocket>=0.10",
     "gpsdclient>=1.3",
     "pyserial>=3.5",
+    # GPIO: install via apt (sudo apt install python3-rpi.gpio)
+    # Not listed here because pip versions require compilation
 ]
 
 [project.optional-dependencies]