feat: adapt CustomSelect styles to native select ::picker pseudo-element

[Copilot]

Proposed changes

Applies CustomSelect component styling to native <select> elements using the new CSS ::picker(select) pseudo-element for consistent design system appearance across custom and native implementations.

Changes

• ::picker(select) styling: Adapted positioning, z-index, shadows, sizing, overflow, and max-height from .db-custom-select-dropdown and .db-custom-select-list
• option element styling: Applied padding, backgrounds, border-radius, hover/active states, and checked font-weight from .db-custom-select-list-item
• Added @use helpers import for hover/active mixins

Styling activates within @supports (appearance: base-select) block, ensuring progressive enhancement.

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

The native select picker now inherits design system styling (elevation, spacing, interactive states) matching the custom select component when browsers support appearance: base-select. Falls back to default browser styling otherwise.

❤️ Thank you!

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