Refactor magazine link visibility based on user authentication

[natlibfi-psams]

Description

This pull request updates how the Magazines section is handled for authenticated and anonymous users.

User Authentication and Navigation:

• The HeaderLinks component now conditionally displays the Magazines navigation button: authenticated users are linked to /magazines, while anonymous users are linked to /magazines-preview, both labeled simply as "Magazines".

Configuration and Environment Handling:

• The getMagazineAllowedOrigin function now robustly validates the NEXT_PUBLIC_MAGAZINES_ORIGIN environment variable, falling back to default origins if the variable is invalid or missing.
  User Context and Token Handling:

• The magazines index page ([library]/magazines/index.tsx) re-enables the use of the useUser hook to retrieve the actual user token, replacing the previous mock token implementation.

How Can This Be Tested?

• This change cannot be tested visually

• Tested locally
• Tested on a mobile device
• Tested with Firefox
• Tested with Chrome
• Tested with Edge
• Tested with Safari

Checklist:

• I have updated the documentation accordingly.
• All new and existing tests passed.
• Review added atleast to E-kirjasto maintainers +1

[natlibfi-burger]

ok