Explain why project feature visibility selects are disabled

What does this MR do and why?

In Project Settings > General > Visibility, the per-feature access-level selects are disabled with no explanation when the project is private (there is no less-visible level to pick) or, for repository-dependent features, when the repository is disabled. Users hit a greyed-out control and have no way to learn why (issue, diagnosis confirmed by the PM in the thread).

This adds a tooltip to the select's wrapper naming the cause:

• Project is private: "Feature visibility cannot be changed while the project is private."
• Repository disabled: "This feature requires the repository to be enabled."

A feature that is simply toggled off gets no tooltip; the adjacent toggle already conveys that. Following the Pajamas tooltip accessibility guidance, the wrapper becomes focusable (tabindex="0" plus a focus ring) only while a reason is shown, so keyboard users can see it; disabled form controls cannot receive focus themselves. Same pattern as the role listbox in app/assets/javascripts/organizations/users/components/user_details_drawer.vue.

ProjectFeatureSetting has a single consumer (settings_panel.vue). The Pages row intentionally stays without a reason: a private project can still offer public Pages, so its select is not locked by privacy.

References

• Related to #503680 (closed)

Screenshots or screen recordings

State	Before	After
Private project (hover a locked select)
Repository turned off (hover a dependent select)

How to set up and validate locally

1. Create or use a private project, visit Settings > General > Visibility, project features, permissions.
2. Hover or keyboard-focus any disabled feature-visibility select (for example Issues): the tooltip explains the project is private.
3. Set Repository access to "No one": hover the Merge requests or CI/CD select: the tooltip explains the repository is required.
4. Make the project public: selects enable, no tooltip, wrapper not focusable.

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