[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 to Reproduce

1. Open any project in app.plane.so
2. Go to Work Items
3. Click "..." (three-dot menu) on any work item
4. Observe the popup menu boundary against the background

Environment