Add jupyterlite-pyodide-kernel to fix missing Python kernel in demo

Bumping jupyterlite from the old 0.1.0rc0 pin to >=0.8.1 dropped the
Python kernel: in 0.1.0rc0 the jupyterlite meta-package hard-depended on
jupyterlite-pyodide-kernel, but that's been split into an optional
package since, so the demo build had no kernel to start ("Start Kernel"
greyed out, only "No Kernel" selectable).

Verified locally: jupyter lite build now bundles
@jupyterlite/pyodide-kernel-extension and sets defaultKernelName=python.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: kolibril13

.jupyterlite/requirements.txt

@@ -5,6 +5,7 @@ scikit-image>=0.26.0
 
 #------ JupyerLite install -------
 jupyterlite>=0.8.1
+jupyterlite-pyodide-kernel>=0.8.2
 
 #------ JupyerLab install -------
 ipywidgets>=8.1.8