[TASK] Expose immutable, canMount, and canUpdate fields in Edit Secret modal

[jenny-s51]

Certification

• I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

Description

Follow-up to the Create Secret modal implementation #751. Add support for editing the immutable, canMount, and canUpdate fields when editing existing secrets. These fields are currently present in the SecretsSecretListItem type but are not exposed in the UI.

Acceptance Criteria

• Create Edit Secret modal component (or extend existing modal to support edit mode)
• Add form controls for immutable, canMount, and canUpdate boolean fields
• Pre-populate form fields with existing secret values when editing
• Update secret via stub API with modified field values
• Validate that immutable secrets cannot be updated (if applicable per API constraints)
• Verify UI follows same patterns and styling as Create Secret modal