[feature]: Improve Popup Menu Border/Shadow Visibility for Better UI Separation

Is there an existing issue for this? • I have searched the existing issues Summary Enhancement Description When clicking the "..." (three-dot) menu on a work item, the popup menu appears without a clearly visible boundary. The popup blends into the background page, making it difficult to distinguish where the page ends and the popup begins. Why should this be worked on? This affects all users who use the "..." context menu on work items. When the popup has no clear boundary, users — especially those with low vision — cannot easily tell the popup is a separate layer from the background page. WCAG 1.4.11 (Non-text Contrast) requires UI component boundaries to have at least 3:1 contrast ratio against adjacent colors. A stronger border or box-shadow would fix this with minimal development effort and no design changes to the menu options themselves. This improves usability for all users and brings the UI closer to WCAG 2.1 AA compliance. Steps t…