Fix matplotlib backend initialization in kernel to restore perfmonitor plots

[OutlyingWest]

• Problem: JumperWrapperKernel loaded jumper_extension before IPKernelApp.init_gui_pylab set MPLBACKEND. The extension imports matplotlib.pyplot early (via visualizer.py), so Matplotlib locked onto a non-inline backend and
  figures fell back to text/plain (<Figure size ...>).
• Change: Add the inline-backend safeguard (MPLBACKEND=module://matplotlib_inline.backend_inline) into _load_jumper_extension, ensuring it is set immediately before importing the extension.

Result: perfmonitor plots rendering issue fixed.

[github-actions]

Docs preview: https://ScaDS.github.io/jumper_wrapper_kernel/pr-3/

[github-actions]

Docs preview: https://ScaDS.github.io/jumper_wrapper_kernel/pr-3/