refactor: extract shared select dropdown/list styles into central SCSS placeholders

[Copilot]

Proposed changes

Eliminates code duplication between CustomSelect components and native select ::picker pseudo-element by extracting common styles into shared SCSS placeholders.

Created: packages/components/src/styles/internal/_select-dropdown-components.scss

• %select-dropdown-container - Dropdown positioning, shadow, sizing, background
• %select-list-container - Overflow behavior and max-height calculation
• %select-list-item - Base item styles (padding, background, border-radius)
• %select-list-item-interaction - Hover/active states

Updated components to extend placeholders:

• select.scss - ::picker(select) and option elements
• custom-select-dropdown.scss - Dropdown container
• custom-select-list.scss - List container
• custom-select-list-item.scss - List items

Reduces ~50 lines of duplicated styles while ensuring consistency across both implementations.

resolves #3654

Types of changes

• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improvements to existing components or architectural decisions)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Further comments

Single source of truth for select dropdown/list styling. Future modifications only need to occur in one location.

❤️ Thank you!

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