feat: increase namespace dropdown panel height for better UX (#6405)

[Antonio-Caiazzo]

What this PR does

Increases the dropdown panel height for the namespace selector to 600px using ::ng-deep .mat-select-panel. This improves usability in environments with many namespaces, allowing more options to be visible without excessive scrolling.

Why it's needed

Fixes #6405. The default dropdown height is too restrictive when handling clusters with a large number of namespaces, making selection inconvenient.

Implementation Notes

• The change was made in style.scss of the namespace selector component.
• !important was used because Angular Material styles are deeply scoped and cannot be overridden otherwise.
• ::ng-deep was necessary to penetrate view encapsulation and apply the rule globally.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Antonio-Caiazzo / name: Antonio Caiazzo (3e4c400, 38f6257)

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