refactor(labextension): replace Kale toolbar polling with reactive state handling

[Sayan4496]

Summary

This PR replaces the polling mechanism previously used to detect the Kale enable state in the JupyterLab toolbar.

Previously the toolbar relied on a setInterval loop to repeatedly check whether Kale was enabled. This approach introduced unnecessary polling in a React-based UI.

This change introduces a reactive callback mechanism that notifies the toolbar whenever the Kale state changes.

Changes

• Removed setInterval polling from kaleToolbar.ts
• Introduced a reactive onKaleStateChange callback in KubeflowKaleLeftPanel
• Toolbar buttons now update instantly when Kale is enabled or disabled
• Simplified state synchronization between the Kale panel and the toolbar

Benefits

• Removes unnecessary polling
• Improves frontend architecture
• Aligns with React’s event-driven state updates
• Makes future UI refactors easier

Testing

• Verified that Compile and Run toolbar buttons update immediately when Kale is enabled/disabled
• Confirmed existing functionality remains unchanged

Closes #649

[google-oss-prow]

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[Sayan4496]

@ederign this PR implements the reactive state handling discussed in #649 by removing the polling mechanism from the toolbar. Please let me know if any improvements are needed.

[Sayan4496]

Hi @jesuino,
Could you please review this PR when you get a chance?
It replaces polling with reactive state handling for the Kale toolbar.

Thanks!