[FIX] side_panel: autocomplete dropdown transparency issue on scroll

[rmbh-odoo]

Description:

Current behavior before PR:

• In commit 2f73242, the bg-white class was removed from the autocomplete search bar.
• This made the search bar transparent, causing dropdown items to appear behind it and break the UI.

Desired behavior after PR is merged:

• Added a proper popover background for the search bar in addDomainSelector.
• Ensured consistent styling and prevented items from showing behind the search bar.

Task: 6254807

review checklist

• feature is organized in plugin, or UI components
• support of duplicate sheet (deep copy)
• in model/core: ranges are Range object, and can be adapted (adaptRanges)
• in model/UI: ranges are strings (to show the user)
• undo-able commands (uses this.history.update)
• multiuser-able commands (has inverse commands and transformations where needed)
• new/updated/removed commands are documented
• exportable in excel
• translations (_t("qmsdf %s", abc))
• unit tested
• clean commented code
• track breaking changes
• doc is rebuild (npm run doc)
• status is correct in Odoo

[robodoo]