fix(labextension): don't block JupyterLab startup with async kernel init

[Copilot]

@lumino/application awaits all autoStart plugin activate() promises before calling shell.attach() (which creates #main). Kale's activate() was async and awaited kernel creation + backend checks (~2–100s), so #main never appeared within browser_check's 100s timeout.

Changes

• Deferred kernel init: kernel work is launched as a background IIFE (initializationPromise) — activate() returns immediately without blocking shell.attach()
• SettingsAwareLeftPanel is async-aware: subscribes to initializationPromise via useEffect/React state; renders null until the kernel is ready, then renders the full panel
• Preserved layout-restoration timing: lab.started.then() is now synchronous — widget is created and restorer.add() is called before lab.restored fires, keeping layout persistence across page reloads intact
• Resume-notebook logic: lab.restored.then() still awaits initializationPromise so nb.resume_notebook_path only runs once the kernel is available
• Removed erroneous throw error in the log.setup_logging catch block that would have caused activate() to reject and crash JupyterLab startup

// Before: activate() is async — blocks shell.attach()
async function activate(...): Promise<IKubeflowKale> {
  const kernel = await NotebookUtils.createNewKernel(); // ← blocks for seconds
  const backend = await getBackend(kernel);             // ← blocks more
  ...
}

// After: activate() returns immediately
function activate(...): Promise<IKubeflowKale> {
  const initializationPromise = (async () => {
    const kernel = await NotebookUtils.createNewKernel(); // runs in background
    const backend = await getBackend(kernel);
    return { kernel, backend };
  })();

  // Widget created synchronously; renders null until initializationPromise resolves
  lab.started.then(() => {
    kalePanelWidget = ReactWidget.create(<SettingsAwareLeftPanel />);
    restorer.add(kalePanelWidget, kalePanelWidget.id); // correct timing preserved
  });
  ...
}

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