Fixes #50835

Problem :

The "View AI Settings" button on the Welcome page was always rendered regardless of the disable_ai setting. This made it visible (and non-functional) for users who had AI disabled, which was confusing.

Fix :

• Adds an optional visibility: Option<fn(&App) -> bool> predicate field to SectionEntry
• At render time, Section::render uses filter_map to skip entries whose predicate returns false.
• The "View AI Settings" entry is given a predicate that checks !DisableAiSettings::get_global(cx).disable_ai, matching the same pattern used in title_bar.rs and quick_action_bar.rs.
• All other entries have visibility: None, meaning they are always shown — no behaviour change for them.

Video :

Release Notes:

• Fixed the "View AI Settings" button being visible on the Welcome page despite AI features being disabled in settings.