don't disclose saved subscription key

[orthagh]

Following a discussion at Caen offices, I mainly removed the possibility to view a subscription key when it's already saved in GLPI configuration. Some people use it to copy advanced key from one instance to another.
It's always possible to retrieve the key in database, decrypt it with GLPI_KEY, but it requires more steps and more knowledge.
And I reworked a bit the page. The latter is rather simple, and the change should be OK even for 11.0/bugfixes.

Screenshots (if appropriate):

[Copilot]

Pull request overview

This PR prevents admins from viewing an already-saved GLPI Network registration/subscription key in the UI, while reworking the setup page layout and adding a server-side “remove key” action.

Changes:

• Reworks the GLPI Network setup Twig template to only allow entering a key when none is saved, and adds a “Remove registration key” button.
• Adds a reset_registration_key handler in front/config.form.php to clear the saved key.
• Updates Marketplace section UI (titles, helper text, buttons).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
templates/pages/setup/general/glpinetwork_setup.html.twig	Reworks the form/UI and hides the saved registration key; adds remove-key UI.
front/config.form.php	Adds POST handler to clear the saved registration key.