Add ui.disable_camera config and gate uStreamer startup on it

[Copilot]

This adds a disable_camera option under the ui section in cosmos.conf, defaulting to False. When enabled, the ustreamer init script exits without starting the camera streaming service.

• Config surface

  • Adds ui.disable_camera = False to the default config shipped by config-manager.
  • Registers disable_camera in the config validator so user overrides are accepted and normalized with the existing config flow.

• uStreamer startup behavior

  • Reads ui.disable_camera via config-manager in the init script.
  • Short-circuits service startup when the setting is True, leaving the service disabled through configuration rather than package/image changes.

• Implementation shape

  • Keeps the change localized to the existing config-manager and SysV init integration.
  • Preserves current behavior unless the new option is explicitly enabled.

if [ "$(config-manager ui disable_camera)" = "True" ]; then
    echo "Camera disabled in configuration; skipping ustreamer startup."
    exit 0
fi