fix: show error notification when cluster deletion fails

[XxSURYANSHxX]

Summary

@illume @joaquimrocha This PR fixes a bug where errors from deleteCluster() were silently swallowed and the user was redirected to home with no feedback. This resolves the TODO comments in ClusterContextMenu and SettingsCluster by dispatching an error notification via the existing Redux notification system.

Related Issue

Fixes #4928

Changes

• Updated frontend/src/components/App/Home/ClusterContextMenu.tsx to dispatch an error notification when cluster deletion fails
• Updated frontend/src/components/App/Settings/SettingsCluster.tsx with the same error notification fix
• Removed the // TODO: create notification with error message comments in both files

Steps to Test

1. Add a dynamic cluster or kubeconfig-based cluster in Headlamp
2. Attempt to delete the cluster via the ⋮ context menu on the Home page while the cluster is already unavailable on the backend (or simulate a failure)
3. Observe that an error notification now appears in the notification bell instead of a silent redirect to /
4. Repeat steps 1–3 from the cluster Settings page (Settings > Cluster > Remove Cluster button)

Screenshots (if applicable)

Before: user is silently redirected to home with no feedback.
After: an error notification appears in the notification bell with the cluster name and error message.

Notes for the Reviewer

• This fix handles all errors from deleteCluster(), not just Not Found, which is a superset of what the original TODO described as this is intentional since any failure should be surfaced to the user.
• The notification message uses the existing i18n t() function, so please check translation key consistency if needed.
• No new dependencies or infrastructure were added; this uses the existing Notification class and setNotifications Redux action already in the codebase.

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