Disable Apply button on Mentors page when registration is closed

[IrynaLypnyk]

Description

Update the Mentors page so the “Apply for this mentor” button reflects the mentorship registration state.

The existing Mentor Filter API should remain unchanged. No new API or frontend filtering logic is required.

Expected behaviour

• If IS_REGISTRATION_OPEN = false, disable the “Apply for this mentor” button for all mentors.
• If IS_REGISTRATION_OPEN = true, keep the current behaviour: the UI/API flow shows only active mentors for the current cycle.
• Do not add additional mentor filtering on the frontend.

Scope

Frontend only.

Acceptance criteria

• “Apply for this mentor” is disabled when IS_REGISTRATION_OPEN is false.
• Existing mentor filters continue to work as before.
• No new API changes are introduced.
• Mentors page renders correctly.

[lulu-cao]

This issue was resolved in #292 and can be closed.