fix(ui-client): remove invalid transparent color token from PageHeader

[roy-abir05]

Proposed changes (including videos or screenshots)

The PageHeader component was previously passing the string 'transparent' to the borderBlockEndColor prop. This is not a valid color token in the Fuselage design system and caused an invalid color: transparent error in the browser console.

This PR fixes the issue by:

1. Passing undefined instead of 'transparent' when no border color is required.
2. Conditionally setting borderBlockEndWidth to 'none' when the border is hidden. This prevents the browser from rendering a default-colored border (visible as a white line) when a width is present but the color is undefined.

Screenshots:
Before:

After:

Issue(s)

Fixes #38577

Steps to test or reproduce

1. Log in as any user.
2. Navigate to the Home page (/home).
3. Open Developer Tools (F12) -> Console.
4. Verify: The invalid color: transparent error does not appear.
5. Verify: There is no unwanted border line visible below the header text.

Further comments

This change satisfies Fuselage's strict token requirements by ensuring only valid tokens or undefined are passed to the Box component.

[dionisio-bot]

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[cubic-dev-ai]

No issues found across 2 files

[roy-abir05]

Hi team!
Just a friendly bump on this.
The PR fixes the console warning and visual regression on the Home page header. CI is green and the CLA is signed. Let me know if you need any adjustments or if this is good to go!