Style useradmin panel as modal with panel styling

[wilhel1812]

Description

Update the useradmin panel to appear in a modal dialog with the same visual styling as other panels in the application.

Requirements

• Panel should have margin from screen edges
• Apply drop shadow consistent with other panels
• Use rounded corners matching panel styling
• Reuse the same CSS classes used for other panels
• Ensure modal behavior (overlay, focus management, etc.)

Acceptance Criteria

• Useradmin panel appears in a modal overlay
• Modal uses identical CSS/styling as other panels
• Panel has proper spacing from viewport edges
• Drop shadow and rounded corners match existing panel design

[wilhel1812]

Implemented in staging (commit 4ee4023). Settings panel now renders as modal with panel styling (border, shadow, rounded corners, margin). On mobile (<=980px), it renders full-screen without border/shadow/rounded corners.

Live on staging: https://staging.linksim.link (v0.18.0-beta+4ee4023a)