select, combobox: Use popover background for dropdown menus (#2574)

Select and Combobox paint their dropdown menu panel with
`tokens.background`,
while PopupMenu and Popover use `tokens.popover`. With a theme that has
a
translucent background color (transparent window setups), open select
menus
become see-through and hard to read while every other floating panel
stays
opaque.

This paints both menus with `tokens.popover` and its paired
`popover_foreground`, the same pair `popover_style()` uses, so they
match the
rest of the popups. In the default themes `popover` falls back to
`background`, so nothing changes visually there. No API changes.

## Screenshot

| Before                       | After                       |
| ---------------------------- | --------------------------- |
| <img width="512" alt="image"
src="https://github.com/user-attachments/assets/3492ed62-7f32-4c54-9118-f70ba7c896f7"
/> | <img width="512" alt="image"
src="https://github.com/user-attachments/assets/58ca615f-0dac-4331-9e54-cabd17804752"
/> |

## How to Test

Run `cargo run` and open the Select and Combobox stories, menus should
look
unchanged with the default themes (`background` and `popover` are the
same
color there). To see the fix, set the theme's `background` to a
translucent
color and open a select menu: it stays opaque instead of showing the
content
behind it.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI generated code
(If any) is accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [x] Tested macOS, Windows and Linux platforms performance (if the
change is platform-specific)

Co-authored-by: Clément Ranouch <69421356+Clems54120@users.noreply.github.com>