ui: Use popover background for dropdown menus

[sh4den]

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

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

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