### Fixes #31865

### Problem
When navigating a picker (command palette, file finder, etc.) with the
keyboard and then hovering with the mouse, two items would appear
highlighted at the same time - one from the keyboard selection
ghost_element_selected background and one from the CSS style
ghost_element_hover background. This happened because GPUI's CSS hover
is applied at paint time independently from the Rust selection state.

### Solution
Added a keyboard_navigated: bool flag to [Picker] to track whether the
last navigation was via keyboard or mouse.

When a keyboard navigation action fires (↑, ↓, Home, End, etc.),
`keyboard_navigated` is set to true. A transparent
`block_mouse_except_scroll` overlay div is added on top of each list
item, preventing the CSS style from firing on `ListItem` children.

When the mouse moves over any item, the overlay's `on_mouse_move`
handler fires, clears keyboard_navigated, moves the selection to that
item, and re-renders - removing all overlays and restoring normal mouse
hover behavior.
The `on_hover` handler also moves the selection to whichever item the
mouse enters, so mouse navigation is fully functional.
Clicks work through the overlay since `block_mouse_except_scroll` only
affects hover hit-testing, not click events on the parent div.

### Behavior
Matches VS Code / IntelliJ — keyboard navigation suppresses hover
highlights until the mouse moves, at which point normal hover resumes.
Only one item is ever highlighted at a time.

### Video

[Screencast from 2026-03-05
19-20-16.webm](https://github.com/user-attachments/assets/7a8a543a-95f3-481f-b59d-171604e5f883)

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Co-authored-by: Nathan Sobo <nathan@zed.dev>