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 :
[Screencast from 2026-03-06
20-18-43.webm](https://github.com/user-attachments/assets/cbfab423-3ef3-41dd-a9ab-cbae055eef6e)


Release Notes:

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